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A first language (also mother tongue, native language, arterial language, or L1) often means the language that a person learns first. It helps one understand words and concepts in the style of that language.
Sometimes, first language means the language that a person speaks best (the second language is then spoken less well than the first language, etc.). Since first language, second language and third language then mean how well one speaks a language, a person can have more than one first language or second language.