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).
Sometimes the term native language is used to indicate a language that a person is as proficient in as a natural-born 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 her 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 (see language attrition).