A first language, native language, arterial language, mother tongue, or L1 in linguistics, is the first language that a person learned. In terms of that view, the person is defined as a native speaker of the first language, although one may also be a native speaker of more than one language if all of the languages were learned without formal education, such as through cultural immersion before puberty. Often a child learns the basics of the first language(s) from family.

Terms not standardised
The terms first, native, and mother can also be misleading, regardless of their definitions. It is quite possible that the first language learned is no longer a speaker's dominant language. Young immigrant children, whose families have moved to a new linguistic environment may lose, in part or in totality, the language they first acquired.
Good skills in one's native languages are essential for further learning, as a native language is thought to be a base of thinking, however, this is highly controversial. Incomplete first language skills often make learning other languages difficult. Native language has therefore a central role in education.

Terms can be misleading
The term "mother tongue" should not be interpreted to mean that it is the language of one's mother. In some paternal societies, the wife moves in with the husband and thus may have a different first language, or dialect, than the local language of the husband. Yet their children usually only speak their local language. Only a few will learn to speak their mothers' languages like natives. Mother in this context probably originated from the definition of mother as source, or origin; as in mother-country or land.
In some countries such as Kenya and India, "mother tongue" is used to indicate the language of one's ethnic group (ethnic tongue), in both common and journalistic parlance (e.g. 'I have no apologies for not learning my mother tongue' [2]), rather than one's first language. A similar usage of the term was employed in Ireland in the early-to-mid twentieth century, with Irish being referred to as the "mother tongue" of all Irish people, even of those whose first language was English.