Original: 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.
Simplified: A first language (also mother tongue, native language, arterial language, or L1) is the language a person first learns.