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).
International Mother Language Day Monument in Sydney, Australia, unveiling ceremony, 19 February 2006
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.