A first language (also native language, mother tongue, arterial language, or L1) is the language(s) a person has learned from birth or within the critical period, or that a person speaks the best and so is often the basis for sociolinguistic identity. In some countries, the terms native language or mother tongue refer to the language of one's ethnic group rather than one's first language. Sometimes, there can be more than one mother tongue, when the child's parents speak different languages. Those children are usually called bilingual.

By contrast, a second language is any language that one speaks other than one's first language.

Sometimes the term native language is used to indicate a language that a person is as proficient in as a native individual of that language's "base country", or as proficient as the average person who speaks no other language but that language.^{[citation needed]}

Sometimes the term mother tongue or mother language is used for the language that a person learnt 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.