An eponym is a person, place, or thing after whom or after which something is named, or believed to be named. The adjectives derived from eponym include eponymous and eponymic. For example, Elizabeth I of England is the eponym of the Elizabethan era, and "the eponymous founder of the Ford Motor Company" refers to Henry Ford. Recent usage, especially in the recorded-music industry, also allows eponymous to mean "named after its central character or creator".    
Periods have often been named after a ruler or other influential figure:
- One of the first recorded cases of eponymy occurred in the second millennium BC, when the Assyrians named each year after a high official (limmu).
- In ancient Greece, the eponymous archon was the highest magistrate in classical Athens. Eponymous archons served a term of one year which took the name of that particular archon (e.g., 594 BC was named for Solon). Later historians provided yet another case of eponymy by referring to the period of fifth-century Athens as The Age of Pericles after its most influential statesman Pericles.
- In Ptolemaic Egypt, the head priest of the Cult of Alexander and the Ptolemies was the eponymous priest after whom years were named.
- The Hebrew Bible explains the origins of peoples through individuals who bear their name. Jacob is renamed "Israel" (Gen 35:9) and his sons (or grandsons) name the original 12 tribes of Israel, while Edomites (Gen. 25:30), Moabites and Ammonites (Gen. 19:30-38), Canaanites (Gen. 9:20-27) and other tribes (the Kenites named after Cain Gen. 4:1-16) are said to be named for other primal ancestors bearing their name. In most cases, the experiences and behavior of the ancestor is meant to indicate the characteristics of the people who take their name.
- In ancient Rome, one of the two formal ways of indicating a year was to cite the two annual consuls who served in that year. For example, the year we know as 59 BC would have been described as "the consulship of Marcus Calpurnius Bibulus and Gaius Julius Caesar" (although that specific year was known jocularly as "the consulship of Julius and Caesar" because of the insignificance of Caesar's counterpart). Under the empire, the consuls would change as often as every two months, but only the two consuls at the beginning of the year would lend their names to that year.
- During the Christian era, itself eponymous, many royal households used eponymous dating by regnal years. The Roman Catholic Church, however, eventually used the Anno Domini dating scheme based on the birth of Christ on both the general public and royalty. The regnal year standard is still used with respect to statutes and law reports published in some parts of the United Kingdom and in some Commonwealth countries (England abandoned this practice in 1963): a statute signed into law in Canada between February 6, 1994 and February 5, 1995 would be dated 43 Elizabeth II, for instance.[ citation needed]
- Government administrations may become referred to eponymously, such as Kennedy's Camelot and the Nixon Era.
- British monarchs have become eponymous throughout the English-speaking world for time periods, fashions, etc. Elizabethan, Georgian, Victorian, and Edwardian are examples of these.
- Political trends or movements often become eponymous with a government leader. Examples include Jacksonian democracy, Stalinism, Maoism, Obamacare, and Thatcherism.
- In intellectual property law an eponym can refer to a generic trademark or brand name, a form of metonymy, such as aspirin,  heroin  and thermos  in the United States.
- In geography, places and towns can also be given an eponymous name through a relationship to an important figure. Peloponnesus, for instance, was said to derive its name from the Greek hero Pelops. In historical times, new towns have often been named (and older communities renamed) after their founders, discoverers, or notable individuals. Examples include Vancouver, British Columbia, named for explorer George Vancouver; and Prince Albert, Saskatchewan, originally called Isbister's Settlement but renamed after Queen Victoria's husband and consort in 1866.
- In science and technology, discoveries and innovations are often named after the discoverer or a figure influential in their advance. Examples are Avogadro's number, the Diesel engine, meitnerium, Alzheimer's disease, and the Apgar score. For a different view of the process see Stigler's law of eponymy.
- In art:
- Plays, books, and other forms of entertainment may have eponymous names, such as the ancient Greek epic The Odyssey, derived from its principal character, Odysseus, and the novel Robinson Crusoe.
- The term is also applied to music, usually with regard to record titles, where it is prevalent and leads to confusion. For example, Bad Company's first album was entitled Bad Company and contained a popular song named " Bad Company". Parodying this, the band R.E.M. titled a 1988 compilation album Eponymous.
- In tribal antiquity, both in ancient Greece and independently among the Hebrews, tribes often took the name of a legendary leader (as Achaeus for Achaeans, or Dorus for Dorians). The eponym gave apparent meaning to the mysterious names of tribes, and sometimes, as in the Sons of Noah, provided a primitive attempt at ethnology as well, in the genealogical relationships of eponymous originators.
- In zoology, an animal name that includes the name of a person is properly called an eponym. 
- Because proper nouns are capitalized in English, the usual default for eponyms is to capitalize the eponymous part of a term. When used as proper adjectives they are normally capitalized, for example Victorian, Shakespearean, and Kafkaesque.  
- However, some eponymous adjectives and noun adjuncts are nowadays entered in many dictionaries as lowercase when they have evolved a common status, no longer deriving their meaning from the proper-noun origin.  For example, Herculean when referring to Hercules himself, but often herculean when referring to the figurative, generalized extension sense;  and quixotic and diesel engine [lowercase only].   For any given term, one dictionary may enter only lowercase or only cap, whereas other dictionaries may recognize the capitalized version as a variant, either equally common as, or less common than, the first-listed styling (marked with labels such as "or", "also", "often", or "sometimes"). The Chicago Manual of Style, in its section "Words derived from proper names",  gives some examples of both lowercase and capitalized stylings, including a few terms styled both ways, and says, "Authors and editors must decide for themselves, but whatever choice is made should be followed consistently throughout a work."
- When the eponym is used together with a noun, the common-noun part is not capitalized (unless it is part of a title or it is the first word in a sentence). For example, in Parkinson disease (named after James Parkinson), Parkinson is capitalized, but disease is not. In addition, the adjectival form, where one exists, is usually lowercased for medical terms (thus parkinsonian although Parkinson disease),  and gram-negative, gram-positive although Gram stain.  Uppercase Gram-positive or Gram-negative however are also commonly used in scientific journal articles and publications.    In other fields, the eponym derivative is commonly capitalized, for example, Newtonian in physics,   and Platonic in philosophy (however, use lowercase platonic when describing love).  The capitalization is retained after a prefix and hyphen, e.g. non-Newtonian. 
For examples, see the comparison table below.
- English can use either genitive case or attributive position to indicate the adjectival nature of the eponymous part of the term. (In other words, that part may be either possessive or non-possessive.) Thus Parkinson's disease and Parkinson disease are both acceptable. Medical dictionaries have been shifting toward nonpossessive styling in recent decades.  Thus Parkinson disease is more likely to be used in the latest medical literature (especially in postprints) than is Parkinson's disease.
- American and British English spelling differences may apply to eponyms. For example, British style would typically be caesarean section, which is also found in American medical publications, but cæsarean section (with a digraph) is sometimes seen in (mostly older) British writing, and cesarean is preferred by American dictionaries and some American medical works. 
|Prevalent dictionary styling today||Stylings that defy prevalent dictionary styling||Comments|
|Addison disease ||*Addison Disease
|Allemann syndrome ||*Allemann Syndrome
cesarean also cesarian [but no cap variant] 
cesarean, "often capitalized" or caesarean also cesarian or caesarian 
|More information on this word's orthographic variants is at Wiktionary: caesarean section.|
darwinism [only] 
Darwinian [only]  
Darwinism [only]  
Darwinist [only]  
|diesel (n/adj/vi) [no cap variant]
diesel engine  
dieseling  
dieselize, dieselization 
draconian often Draconian 
eustachian often Eustachian 
eustachian tube [only] 
eustachian tube often Eustachian tube 
eustachian tube or Eustachian tube 
fallopian often Fallopian 
fallopian tube [only] 
fallopian tube often Fallopian tube 
fallopian tube also Fallopian tube 
Marxist [only]  
 or Mendelian [only]
mendelian inheritance [only]  or Mendelian inheritance [only] 
Mendel's laws  
|Newtonian [only]  ||*newtonian|
parkinsonian [only]  
parkinsonian tremor 
Parkinson disease [only] 
Parkinson's disease [only] 
|quixotic [only]  ||*Quixotic|
roman numerals 
|AMA Manual of Style lowercases the terms roman numerals and arabic numerals. MWCD enters the numeral sense under the headword Roman but with the note "not cap" on the numeral sense. |
By person's name
- Astronomical objects
- Cartoon characters
- Chemical elements
- Colleges and universities
- Human anatomical parts
- Mathematical theorems
- Medical signs
- Medical treatments
- Places and political entities
- Prizes, awards and medals
- Scientific constants
- Scientific equations
- Scientific laws
- Scientific phenomena
- Scientific units
- Sports terms
- Surgical procedures
- Trademarks or brand names
- Archetypal name
- Eponymous hairstyles
- False etymology
- Genericized trademark
- Medical eponyms
- Name reaction
- Stigler's law of eponymy
- Territorial designation
- "eponym". Dictionary.com. Dictionary.com LLC. Retrieved 30 December 2014.
- "eponym". Merriam-Webster Online Dictionary. Merriam-Webster. Retrieved 30 December 2014.
- "eponymous". Dictionary.com. Dictionary.com LLC. Retrieved 30 December 2014.
- "eponymous". Merriam-Webster Online Dictionary. Merriam-Webster. Retrieved 30 December 2014.
- "Orion Spacecraft - Nasa Orion Spacecraft". aerospaceguide.net.
- Bayer Co. v. United Drug Co., 272 F. 505 (S.D.N.Y. 1921), Berkman Center for Internet & Society at Harvard University, accessed March 25th, 2011
- "Online Etymology Dictionary". etymonline.com.
- King-Seeley Thermos Co. v. Aladdin Indus., Inc., 321 F.2d 577 ( 2d Cir. 1963); see also this PDF Archived 2006-02-09 at the Wayback Machine.
- Beolens, Bo; Watkins, Michael; Grayson, Michael (2014). The Eponym Dictionary of Birds. Bloomsbury Publishing. ISBN 1472905741.
- Waddingham, Anne (28 August 2014). New Hart's Rules: The Oxford Style Guide. OUP Oxford. p. 105. ISBN 978-0199570027.
- Marthus-Adden Zimboiant. No Grammar Tears 1. pp. 256&ndash, 257. ISBN 9781491800751.
- Merriam-Webster (1993), Merriam-Webster's Collegiate Dictionary (10th ed.), Springfield, Massachusetts, USA: Merriam-Webster, ISBN 978-0-87779-707-4
- Houghton Mifflin (2000), The American Heritage Dictionary of the English Language (4th ed.), Boston and New York: Houghton Mifflin, ISBN 978-0-395-82517-4
- University of Chicago (1993). The Chicago Manual of Style (14th ed.). Chicago: University of Chicago Press. § 7.49, pp. 253–254. ISBN 0-226-10389-7.
- Lorraine Villemaire, Doreen Oberg (29 December 2005). Grammar and Writing Skills for the Health Professional (2nd Revised ed.). Delmar Cengage Learning. p. 167. ISBN 978-1401873745.
- Centers for Disease Control and Prevention. Emerging Infectious Diseases Journal Style Guide. Preferred Usage
- Lisa Brown, Julie M. Wolf, Rafael Prados-Rosales & Arturo Casadevall (2015). "Through the wall: extracellular vesicles in Gram-positive bacteria, mycobacteria and fungi". Nature Reviews Microbiology. 13: 620–630. doi: 10.1038/nrmicro3480. PMC 4860279. PMID 26324094.
- Kristen L. Mueller (12 June 2015). "Detecting Gram-negative bacteria". Science. 348 (6240): 1218. doi: 10.1126/science.348.6240.1218-o.
- "Gram-positive". Dictionary.com.
- "Newtonian". Merriam-Wester.
- "New·ton". The American Heritage Dictionary.
- Iverson, Cheryl (editor) (2007), AMA Manual of Style (10 ed.), Oxford, England: Oxford University Press, ISBN 978-0-19-517633-9, chapter 16: Eponyms.
- Medical Subject Headings (MeSH) of the United States National Library of Medicine (NLM) uses "cesarean section", while the also US-published Saunders Comprehensive Veterinary Dictionary uses "caesarean". The online versions of the Merriam-Webster Dictionary and American Heritage Dictionary list "cesarean" first and other spellings as "variants", an etymologically anhistorical position.
- Elsevier (2007), Dorland's Illustrated Medical Dictionary (31st ed.), Philadelphia: Elsevier, ISBN 978-1-4160-2364-7
- Merriam-Webster (2003), Merriam-Webster's Collegiate Dictionary (11th ed.), Springfield, Massachusetts, USA: Merriam-Webster, ISBN 978-0-87779-809-5
|Look up eponym in Wiktionary, the free dictionary.|
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