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, (for example, 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.
Read more about First Language: Terminology, Significance, On Multilinguality
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“As in private life one differentiates between what a man thinks and says of himself and what he really is and does, so in historical struggles one must still more distinguish the language and the imaginary aspirations of parties from their real organism and their real interests, their conception of themselves from their reality.”
—Karl Marx (1818–1883)