A first language (also native language, mother tongue, arterial language, or L1) is the language or are the languages 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. Children brought up speaking more than one language can have more than one native language, and be bilingual.