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A first language, native language, or mother/father/parent tongue (also known as arterial language or L1) is a language that a person has been exposed to from birth  or within the critical period. In some countries, the term native language or mother tongue refers to the language of one's ethnic group rather than one's first language.  Children brought up speaking more than one language can have more than one native language, and be bilingual or multilingual. By contrast, a second language is any language that one speaks other than one's first language.
One of the more widely accepted definitions of native speakers is that they were born in a particular country raised to speak the language of that country during the critical period of their development,  The person qualifies as a "native speaker" of a language by being born and immersed in the language during youth, in a family in which the adults shared a similar language experience as the child.  Native speakers are considered to be an authority on their given language because of their natural acquisition process regarding the language, as opposed to having learned the language later in life. That is achieved by personal interaction with the language and speakers of the language. Native speakers will not necessarily be knowledgeable about every grammatical rule of the language, but they will have good "intuition" of the rules through their experience with the language. 
Sometimes, the term "mother tongue" or "mother language" is used for the language that a person learned as a child at home (usually from their parents). Children growing up in bilingual homes can, according to this definition, have more than one mother tongue or native language.
In the context of population censuses conducted on the Canadian population, Statistics Canada defines mother tongue as "the first language learned at home in childhood and still understood by the individual at the time of the census."  It is quite possible that the first language learned is no longer a speaker's dominant language. That includes young immigrant children whose families have moved to a new linguistic environment as well as people who learned their mother tongue as a young child at home (rather than the language of the majority of the community), who may have lost, in part or in totality, the language they first acquired (see language attrition).
According to Ivan Illich, the term "mother tongue" was first used by Catholic monks to designate a particular language they used, instead of Latin, when they are "speaking from the pulpit". That is, the "holy mother the Church" introduced this term and colonies inherited it from Christianity as a part of colonialism.  
In some countries, such as Kenya, India, and various East Asian countries, "mother language" or "native language" is used to indicate the language of one's ethnic group in both common and journalistic parlance ("I have no apologies for not learning my mother tongue"), rather than one's first language. Also, in Singapore, "mother tongue" refers to the language of one's ethnic group regardless of actual proficiency, and the "first language" refers to English, which was established on the island under the British Empire, which is the lingua franca for most post-independence Singaporeans because of its use as the language of instruction in government schools and as a working language.
J. R. R. Tolkien, in his 1955 lecture " English and Welsh," distinguishes the "native tongue" from the "cradle tongue." The latter is the language one happens to learn during early childhood, and one's true "native tongue" may be different, possibly determined by an inherited linguistic taste and may later in life be discovered by a strong emotional affinity to a specific dialect (Tolkien personally confessed to such an affinity to the Middle English of the West Midlands in particular).
On 17 November 1999, UNESCO designated 21 February as International Mother Language Day.
The first language of a child is part of the personal, social and cultural identity.  Another impact of the first language is that it brings about the reflection and learning of successful social patterns of acting and speaking.  It is basically responsible for differentiating the linguistic competence of acting. While some argue that there is no such thing as "native speaker" or a "mother tongue," it is important to understand the key terms as well as understand what it means to be a "non-native" speaker and the implications that can have on one's life. Research suggest that while a non-native speaker may develop fluency in a targeted language after about two years of immersion, it can actually take between five and seven years for that child to be on the same working level as their native speaking counterparts. That has implications on the education of non-native speakers. 
The topic of native speaker also gives way to discussion about what exactly bilingualism is. One definition is that a person is bilingual by being equally proficient in both languages. A person who grows up speaking English and begins learning Spanish for four years is not necessarily bilingual unless he speaks the two languages with equal fluency. Pearl and Lambert were the first to test only “balanced” bilinguals—that is, children who are completely fluent in two languages and feel that neither is their “native” language because they grasp the two so perfectly. This study found the following: balanced bilinguals perform significantly better in tasks that require flexibility (they constantly shift between the two known languages depending on the situation/requires constant juggling), more aware of arbitrary nature of language and also that balanced bilinguals choose word associations based on logical rather than phonetic preferences.  
One can have two or more native languages, thus being a native bilingual or indeed multilingual. The order in which these languages are learned is not necessarily the order of proficiency. For instance, if a French-speaking couple have a child who learned French first but then the child grew up in an English-speaking country, the child would likely be most proficient in English. Other examples are in India, Indonesia, the Philippines, Kenya, Malaysia, Singapore, and South Africa, where most people speak more than one language.
The designation "native language," in its general usage, is thought to be imprecise and subject to various interpretations that are biased linguistically, especially with respect to bilingual children from ethnic minority groups. Many scholars[ citation needed] have given definitions of 'native language' based on common usage, the emotional relation of the speaker towards the language, and even its dominance in relation to the environment. However, all of three criteria lack precision. For many children whose home language differs from the language of the environment (the 'official' language), it is debatable which language is one's 'native language'.
- Based on origin: the language(s) one learned first (the language(s) in which one has established the first long-lasting verbal contacts).
- Based on internal identification: the language(s) one identifies with/as a speaker of;
- Based on external identification: the language(s) one is identified with/as a speaker of, by others.
- Based on competence: the language(s) one knows best.
- Based on function: the language(s) one uses most.
The article titled “The Native Speaker: An Achievable Model?” published by the Asian EFL Journal  states that there are six general principles that relate to the definition of "native speaker". The principles, according to the study, are typically accepted by language experts across the scientific field. A native speaker is defined according to the guidelines as this:
- The individual acquired the language in early childhood.
- The individual has intuitive knowledge of the language.
- The individual is able to produce fluent, spontaneous discourse.
- The individual is competent in communication.
- The individual identifies with or is identified by a language community.
- The individual has a dialect accent (including the official dialect).
- Heritage language
- Child of deaf adult
- Human Speechome Project
- Third culture kids
- List of languages by number of native speakers
- Statistical learning in language acquisition
- Native speaker
- Father Tongue hypothesis
- Bloomfield, Leonard. Language ISBN 81-208-1196-8
- "K*The Native Speaker: Myth and Reality By Alan Davies ISBN 1-85359-622-1[ page needed]
- "Who Is An Ideal Native Speaker?! Andisheh Saniei, English Language Department, Science and Research Branch, Islamic Azad University, Tehran, Iran" (PDF). ipedr.com. Archived (PDF) from the original on 2 February 2018. Retrieved 1 February 2018.
- Love, Nigel, and Umberto Ansaldo. "The Native Speaker and the Mother Tongue." Language Sciences 32.6 (2010): 589-93. Print.
- "mother tongue". 2001 census. Archived from the original on 16 September 2008. Retrieved 25 August 2008.[ unreliable source?]
- [Ivan Illich] in Patttanayak, 1981:24 cited in "(M)other Tongue Syndrome: From Breast to Bottle" Archived 30 August 2017 at the Wayback Machine.
- Ivan Illich, "Vernacular Values" Archived 20 July 2016 at the Wayback Machine.
- "Terri Hirst: The Importance of Maintaining a Childs First Language". bisnet.or.id. Archived from the original on 12 March 2016.
- "Archived copy" (PDF). Archived from the original (PDF) on 10 May 2013. Retrieved 17 September 2013.
- Second Language Acquisition Essential Information: Professor J. Cumminshttp://esl.fis.edu/teachers/support/cummin.htm
- "Language Proficiency: Defining Levels Avoids Confusion". Alsintl.com. 26 August 2013. Archived from the original on 17 September 2013. Retrieved 13 November 2013.
- "Archived copy" (PDF). Archived (PDF) from the original on 24 October 2013. Retrieved 21 October 2013.
- Lee, Joseph. "The Native Speaker: An Achievable Model?". Asian EFL Journal. 7 (2).