Definition of Native Language:
-the language that a person has spoken from earliest childhood.
http://www.thefreedictionary.com/native+language
NATIVE LANGUAGE - When used with reference to a person of limited English-speaking ability, means the language normally used by that person, or in the case of a child, the language normally used by the parents of the child. Bilingual Education Act.
From Oxford:
native speaker
• noun a person who has spoken the language in question from earliest childhood.
From Wikipedia:
Mother tongue
The term "mother tongue" should not be interpreted to mean that it is the language of one's mother.
The usage of these terms is far from standardized, however.
Sometimes the term first language is used for the language that the speaker speaks best (his second language then being the language he speaks less well than his first language, etc).
like the terms first language, second language and third language are used to indicate various levels of skill in a language, so that it can be said that a person knows more than one language at first or second language level.
Sometimes the term native language is used to indicate a language that a person is as proficient in as a native inhabitant of that language's base country, or as proficient as the average person who speaks no other language but that language.
Sometimes the term mother tongue or mother language is used for the language that a person learnt at home (usually from his parents). Children growing up in bilingual homes can according to this definition have more than one mother tongue.
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."[3] 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
http://en.wikipedia.org/wiki/First_language
