In linguistics, and particularly phonology, stress or accent is relative emphasis or prominence given to a certain syllable in a word, or to a certain word in a phrase or sentence. This emphasis is typically caused by such properties as increased loudness and vowel length, full articulation of the vowel, and changes in pitch.   The terms stress and accent are often used synonymously in this context, but they are sometimes distinguished. For example, when emphasis is produced through pitch alone, it is called pitch accent, and when produced through length alone, it is called quantitative accent.  When caused by a combination of various intensified properties, it is called stress accent or dynamic accent; English uses what is called variable stress accent.
Since stress can be realised through a wide range of phonetic properties, such as loudness, vowel length, and pitch, which are also used for other linguistic functions, it is difficult to define stress solely phonetically.
The stress placed on syllables within words is called word stress or lexical stress. Some languages have fixed stress, meaning that the stress on virtually any multisyllable word falls on a particular syllable, such as the penultimate (e.g. Polish) or the first. Other languages, like English and Russian, have variable stress, where the position of stress in a word is not predictable in that way. Sometimes more than one level of stress, such as primary stress and secondary stress, may be identified. However, some languages, such as French and Mandarin, are sometimes analyzed as lacking lexical stress entirely.
The stress placed on words within sentences is called sentence stress or prosodic stress. This is one of the three components of prosody, along with rhythm and intonation. It includes phrasal stress (the default emphasis of certain words within phrases or clauses), and contrastive stress (used to highlight an item − a word, or occasionally just part of a word − that is given particular focus).
There are various ways in which stress manifests itself in the speech stream, and these depend to some extent on which language is being spoken. Stressed syllables are often louder than non-stressed syllables, and may have a higher or lower pitch. They may also sometimes be pronounced longer. There are sometimes differences in place or manner of articulation – in particular, vowels in unstressed syllables may have a more central (or " neutral") articulation, while those in stressed syllables have a more peripheral articulation. Stress may be realized to varying degrees on different words in a sentence; sometimes the difference between the acoustic signals of stressed and unstressed syllables are minimal.
These particular distinguishing features of stress, or types of prominence in which particular features are dominant, are sometimes referred to as particular types of accent – dynamic accent in the case of loudness, pitch accent in the case of pitch (although this term usually has more specialized meanings), quantitative accent in the case of length,  and qualitative accent in the case of differences in articulation. These can be compared to the various types of accent in music theory. In some contexts, the term stress or stress accent is used to mean specifically dynamic accent (or as an antonym to pitch accent in its various meanings).
A prominent syllable or word is said to be accented or tonic; the latter term does not imply that it carries phonemic tone. Other syllables or words are said to be unaccented or atonic. Syllables are frequently said to be in pretonic or post-tonic position; certain phonological rules apply specifically to such positions. For instance, in American English, /t/ and /d/ are flapped in post-tonic position.
In Mandarin Chinese, which is a tone language, stressed syllables have been found to have tones realized with a relatively large swing in fundamental frequency, while unstressed syllables typically have smaller swings.  (See also Stress in Standard Chinese.)
Stressed syllables are often perceived as being more forceful than non-stressed syllables. Research has shown, however, that although dynamic accent is accompanied by greater respiratory force, it does not mean a more forceful articulation in the vocal tract.[ citation needed]
Lexical stress, or word stress, is the stress placed on a given syllable in a word. The position of lexical stress in a word may depend on certain general rules applicable in the language or dialect in question, but in other languages, it must be learned for each word, as it is largely unpredictable.
Languages in which position of the stress can usually be predicted by a simple rule are said to have fixed stress. For example, in Czech, Finnish, Icelandic and Hungarian, the stress almost always comes on the first syllable of a word. In Armenian the stress is on the last syllable of a word.  In Quechua, Esperanto, and Polish, the stress is almost always on the penult (second-last syllable). In Macedonian, it is on the antepenult (third-last syllable).
Other languages have stress placed on different syllables but in a predictable way, as in Classical Arabic and Latin (whose stress is conditioned by the structure of the penult). They are said to have a regular stress rule.
Statements about the position of stress are sometimes affected by the fact that when a word is spoken in isolation, prosodic factors (see below) come into play, which do not apply when the word is spoken normally within a sentence. French words are sometimes said to be stressed on the final syllable, but that can be attributed to the prosodic stress that is placed on the last syllable (unless it is a schwa, when it is the second-last) of any string of words in that language. Thus, it is on the last syllable of a word analyzed in isolation. The situation is similar in Standard Chinese. French (some authors add Chinese ) can be considered to have no real lexical stress.
Languages in which the position of stress in a word is less predictable are said to have variable stress like English, Russian, Italian, Portuguese and Spanish. Stress is usually truly lexical and must be memorized as part of the pronunciation of an individual word. In some languages, such as in Spanish, in Portuguese, in Lakota and, to some extent in Italian, stress is even represented in writing using diacritical marks, for example in the Spanish words célebre and celebré. In Russian, diacritical marks are sometimes written for people learning the language, whether as a first or second language.
In such languages with variable stress, stress may be phonemic in that it can serve to distinguish otherwise identical words. For example, the English words insight and incite are distinguished in pronunciation only by the fact that the stress falls on the first syllable in the former and on the second syllable in the latter. Other examples include umschreiben ("rewrite") vs. umschreiben ("paraphrase, outline") in German, за́мок ("castle") vs. замо́к ("lock") in Russian, and ancora ("anchor"), ancora ("more, still, yet") in Italian, and the triple example sábia ("wise woman"), sabia ("know"), sabiá ("type of bird") from Portuguese.
With very few exceptions, English compound words are stressed on their first component. And even such exceptions, for example mankínd,  are instead often stressed on the first component by some people or in some kinds of English.  Sometimes the same components as those of a compound word are used in a descriptive phrase with a different meaning and with stress on both words, but then this descriptive phrase is not usually considered a compound, e.g. bláck bírd (any bird that is black) and bláckbird (a specific bird species) and páper bág (a bag made of paper) and páper bag (very rarely used to mean a bag for carrying newspapers, often also used to mean a bag made of paper). 
Dialects of the same language may have different stress placement. For instance, the English word laboratory is stressed on the second syllable in British English (labóratory often pronounced "labóratry", the second o being silent), but the first syllable in American English, with a secondary stress on the "tor' syllable (láboratory often pronounced "lábratory"). The Spanish word video is stressed on the first syllable in Spain (vídeo) but on the second syllable in the Americas (vidéo). The Portuguese words for Madagascar and the continent Oceania are stressed on the third syllable in European Portuguese (Madagáscar and Oceânia), but on the fourth syllable in Brazilian Portuguese (Madagascar and Oceania).
Some languages are described as having both primary stress and secondary stress. A syllable with secondary stress is stressed relative to unstressed syllables but not as strongly as a syllable with primary stress. As with primary stress, the position of secondary stress may be more or less predictable depending on language. In English, it is not fully predictable, but the different secondary stress of the words organization and accumulation (on the first and second syllable, respectively) is predictable due to the same stress of the verbs órganize and accúmulate. In some analyses, for example the one found in Chomsky and Halle's The Sound Pattern of English, English has been described as having four levels of stress: primary, secondary, tertiary, and quaternary, but the treatments often disagree with one another.
Peter Ladefoged and other phoneticians have noted that it is possible to describe English with only one degree of stress, as long as unstressed syllables are phonemically distinguished for vowel reduction.  They believe that the multiple levels posited for English, whether primary–secondary or primary–secondary–tertiary, are mere phonetic detail and not true phonemic stress, and often, the alleged secondary stress is not characterized by the increase in respiratory activity normally associated with primary stress in English or with all stress in other languages. (For further detail see Stress and vowel reduction in English.)
Prosodic stress, or sentence stress, refers to stress patterns that apply at a higher level than the individual word – namely within a prosodic unit. It may involve a certain natural stress pattern characteristic of a given language, but may also involve the placing of emphasis on particular words because of their relative importance (contrastive stress).
An example of a natural prosodic stress pattern is that described for French above; stress is placed on the final syllable of a string of words (or if that is a schwa, the next-to-final syllable). A similar pattern has been claimed for English (see § Levels of stress above): the traditional distinction between (lexical) primary and secondary stress is replaced partly by a prosodic rule stating that the final stressed syllable in a phrase is given additional stress. (A word spoken alone becomes such a phrase, hence such prosodic stress may appear to be lexical if the pronunciation of words is analyzed in a standalone context rather than within phrases.)
Another type of prosodic stress pattern is quantity sensitivity – in some languages additional stress tends to be placed on syllables that are longer ( moraically heavy).
Prosodic stress is also often used pragmatically to emphasize (focus attention on) particular words or the ideas associated with them. Doing this can change or clarify the meaning of a sentence; for example:
As in the examples above, stress is normally transcribed as italics in printed text or underlining in handwriting.
In English, stress is most dramatically realized on focused or accented words. For instance, consider the dialogue
In it, the stress-related acoustic differences between the syllables of "tomorrow" would be small compared to the differences between the syllables of "dinner", the emphasized word. In these emphasized words, stressed syllables such as "din" in "dinner" are louder and longer.    They may also have a different fundamental frequency, or other properties.
The main stress within a sentence, often found on the last stressed word, is called the nuclear stress. 
In many languages, such as Russian and English, vowel reduction may occur when a vowel changes from a stressed to an unstressed position. In English, unstressed vowels may reduce to schwa-like vowels, though the details vary with dialect (see Stress and vowel reduction in English). The effect may be dependent on lexical stress (for example, the unstressed first syllable of the word photographer contains a schwa //, whereas the stressed first syllable of photograph does not /ˈfoʊtəˌgræf -grɑːf/), or on prosodic stress (for example, the word of is pronounced with a schwa when it is unstressed within a sentence, but not when it is stressed).
Many other languages, such as Finnish and the mainstream dialects of Spanish, do not have unstressed vowel reduction; in these languages vowels in unstressed syllables have nearly the same quality as those in stressed syllables.
Some languages, such as English, are said to be stress-timed languages; that is, stressed syllables appear at a roughly constant rate and non-stressed syllables are shortened to accommodate this. This contrasts with languages that have syllable timing (e.g. Spanish) or mora timing (e.g. Japanese), where syllables or moras are spoken at a roughly constant rate regardless of stress. For details, see Isochrony.
It is common for stressed and unstressed syllables to behave differently as a language evolves. For example, in the Romance languages, the original Latin short vowels /e/ and /o/ have often become diphthongs when stressed. Since stress takes part in verb conjugation, this has produced verbs with vowel alternation in the Romance languages. For example, the Spanish verb volver has the form volví in the past tense but vuelvo in the present tense (see Spanish irregular verbs). Italian shows the same phenomenon but with /o/ alternating with /uo/ instead. This behavior is not confined to verbs; note for example Spanish viento "wind" from Latin ventum, or Italian fuoco "fire" from Latin focum.
An operational definition of word stress may be provided by the stress "deafness" paradigm.  The idea is that if listeners perform poorly on reproducing the presentation order of series of stimuli that minimally differ in the position of phonetic prominence (e.g. [númi]/[numí]), the language doesn't have word stress. The task involves a reproduction of the order of stimuli as a sequence of key strokes, whereby key '1' is associated with one stress location (e.g. [númi]) and key '2' with the other (e.g. [numí]). A trial may be from 2 to 6 stimuli in length. Thus, the order [númi-númi-numí-númi] is to be reproduced as '1121'. It was found that listeners whose native language was French performed significantly worse than Spanish listeners in reproducing the stress patterns by key strokes. The explanation is that Spanish has lexically contrastive stress, as evidenced by the minimal pairs like tópo ("mole") and topó ("met"), while in French, stress does not convey lexical information and there is no equivalent of stress minimal pairs as in Spanish.
The orthographies of some languages include devices for indicating the position of lexical stress. Some examples are listed below:
- In Modern Greek, all polysyllables are written with an acute accent over the vowel of the stressed syllable. (The acute accent is also used on some monosyllables in order to distinguish homographs, as in η ("the") and ή ("or"); here the stress of the two words is the same.)
- In Spanish orthography, stress may be written explicitly with a single acute accent on a vowel. Stressed antepenultimate syllables are always written with this accent mark, as in árabe. If the last syllable is stressed, the accent mark is used if the word ends in the letters n, s, or a vowel, as in está. If the penultimate syllable is stressed, the accent is used if the word ends in any other letter, as in cárcel. That is, if a word is written without an accent mark, the stress is on the penult if the last letter is a vowel, n, or s, but on the final syllable if the word ends in any other letter. However, as in Greek, the acute accent is also used for some words to distinguish various syntactical uses (e.g. té "tea" vs. te a form of the pronoun tú; dónde "where" as a pronoun or wh-complement, donde "where" as an adverb). For more information, see Stress in Spanish.
- In Portuguese, stress is sometimes indicated explicitly with an acute accent (for i, u, and open a, e, o), or circumflex (for close a, e, o). The orthography has an extensive set of rules that describe the placement of diacritics, based on the position of the stressed syllable and the surrounding letters.
- In Italian, the grave accent is needed in words ending with an accented vowel, e.g. città, "city", and in some monosyllabic words that might otherwise be confused with other words, like là ("there") and la ("the"). It is optional for it to be written on any vowel if there is a possibility of misunderstanding, such as condomìni ("condominiums") and condòmini ("joint owners"). See Italian alphabet § Diacritics.
Though not part of normal orthography, a number of devices exist that are used by linguists and others to indicate the position of stress (and syllabification in some cases) when it is desirable to do so. Some of these are listed here.
- In the International Phonetic Alphabet (IPA), primary stress is indicated by a high vertical line before the stressed element, secondary stress by a low vertical line. For example, [sɪˌlæbəfɪˈkeɪʃən] or /sɪˌlæbəfɪˈkeɪʃən/. Extra stress can be indicated by doubling the symbol: ˈˈ◌. Most commonly, the stress mark is placed before the beginning of the stressed syllable, where a syllable is definable. However, it is occasionally placed immediately before the vowel. 
- Linguists frequently mark primary stress with an acute accent over the vowel, and secondary stress by a grave accent. Example: [sɪlæ̀bəfɪkéɪʃən] or /sɪlæ̀bəfɪkéɪʃən/. This has the advantage that it does not require a decision about syllable boundaries.
- In English dictionaries that show pronunciation by respelling, stress is typically marked with a prime mark placed after the stressed syllable: /si-lab′-ə-fi-kay′-shən/.
- In ad hoc pronunciation guides, stress is often indicated using a combination of bold text and capital letters. For example, si-lab-if-i-KAY-shun or si-LAB-if-i-KAY-shun
- In Russian and Ukrainian dictionaries, stress is indicated with an acute accent (´) on a syllable's vowel (example: вимовля́ння) or, in other editions, an apostrophe  just after it (example: гла'сная). Stressing is rare in general texts, but is still used when necessary: compare за́мок (castle) and замо́к (lock). Stress marks are generally used only in materials for foreign learners of the language.
- In Dutch, ad hoc indication of stress is usually marked by an acute accent on the vowel (or, in the case of a diphthong or double vowel, the first two vowels) of the stressed syllable. Compare achterúítgang (deterioration) and áchteruitgang (back exit).
- In Biblical Hebrew, a complex system of cantillation marks is used to mark stress, as well as verse syntax and the melody according to which the verse is chanted in ceremonial Bible reading. In Modern Hebrew, there is no standardized way to mark the stress. Most often, the cantillation mark oleh (part of oleh ve-yored), which looks like a left-pointing arrow above the consonant of the stressed syllable, for example ב֫וקר bóker (morning) as opposed to בוק֫ר bokér (cowboy). This mark is usually used in books by the Academy of the Hebrew Language, and it is available on the standard Hebrew keyboard at AltGr-6. In some books other marks, such as meteg, are used.
- Accent (poetry)
- Accent (music)
- Foot (prosody)
- Initial-stress-derived noun
- Pitch accent (intonation)
- Pitch-accent language
- Syllable weight
- Fry, D.B. (1955). "Duration and intensity as physical correlates of linguistic stress. Journal of the Acoustical Society of America". Journal of the Acoustical Society of America. 27: 765–768.
- Fry, D.B. (1958). "Experiments in the perception of stress". Language and Speech. 1: 126–152.
- G. H. Monrad-Krohn, "The prosodic quality of speech and its disorders (a brief survey from a neurologist's point of view)", Acta Psychiatrica Scandinavica, vol. 22, issue 3-4, pp. 255-269.
- Kochanski, G., Shih, C., Jing, H.; Quantitative Measurement of Prosodic Strength in Mandarin, Speech Communication 41(4), November 2003, doi: 10.1016/S0167-6393(03)00100-6
- Mirakyan, Norayr (2016). "The Implications of Prosodic Differences Between English and Armenian" (PDF). Collection of Scientific Articles of YSU SSS. YSU Press. 1.3 (13): 91–96.
- San Duanmu (2000), The Phonology of Standard Chinese, Oxford University Press, p. 134.
- mankind in the Collins English Dictionary
- mankind in the American Heritage Dictionary
- Ladefoged (1975 etc.) A course in phonetics § 5.4; (1980) Preliminaries to linguistic phonetics p 83
- M. E. Beckman, Stress and Non-Stress Accent, Dordrecht: Foris (1986) ISBN 90-6765-243-1
- R. Silipo and S. Greenberg, Automatic Transcription of Prosodic Stress for Spontaneous English Discourse, Proceedings of the XIVth International Congress of Phonetic Sciences (ICPhS99), San Francisco, CA, August 1999, pages 2351–2354
- G. Kochanski, E. Grabe, J. Coleman and B. Rosner, Journal of the Acoustical Society of America, volume 118, number 2, pages 1038–1054, doi: 10.1121/1.1923349
- Iggy Roca, Thematic Structure: Its Role in Grammar, Walter de Gruyter 1992, p. 80.
- Dupoux, Emmanuel; Peperkamp, Sharon; Sebastián-Gallés, Núria (2001-09-01). "A robust method to study stress "deafness"". The Journal of the Acoustical Society of America. 110 (3): 1606–1618. doi: 10.1121/1.1380437. ISSN 0001-4966.
- Payne, E. M. (2005) "Phonetic variation in Italian consonant gemination", Journal of the International Phonetic Association, 35: 153–181.
- Dalʹ, Vladimir Ivanovich (1903). Tolkovyĭ Slovarʹ Zhivogo Velikorusskago I︠a︡zyka. I. A. Boduėna-de-Kurtene (ed.) (3., ispr. i znachitel'no dop. izd ed.). Sankt-Peterburg: Izd. T-va M.O. Vol'f. p. 4.