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 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. gardless 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 (see language attrition).
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.
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', 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. Also in Singapore, "mother tongue" refers to the language of one's ethnic group regardless of actual proficiency, while the "first language" refers to the English language, which is the lingua franca for most post-independence Singaporeans due to its use as the language of instruction in government schools and as a working language despite it not being a native tongue for most Singaporeans.
J. R. R. Tolkien in his 1955 lecture English and Welsh distinguishes the "native tongue" from the "cradle tongue", the latter being the language one happens to learn during early childhood, while 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).
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, a French-speaking couple might have a daughter who learned French first, then English; but if she grew up in the United States, she is likely to become more proficient in English.
The Brazilian linguist Cleo Altenhofen considers the denomination "mother tongue" in its general usage to be imprecise and subject to various interpretations that are biased linguistically, especially with respect to bilingual children from ethnic minority groups. He cites his own experience as a bilingual speaker of Portuguese language and Riograndenser Hunsrückisch, a German-rooted language brought to southern Brazil by the first German immigrants. In his case, like that of many children whose home language differs from the language of the environment (the 'official' language), it is debatable which language is his 'mother tongue'. Many scholars gave definitions of 'mother tongue' through the years 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 these criteria lack precision.