When Chomsky identified the logical problem of language acquisition which behaviorist approaches could not solve, he focused on theoretical aspects of first language acquisition.
Second language researchers and teachers are applied linguists, and consider different populations.
We distinguish the first (native) language or mother tongue, L1, which is the language a child learns from birth, from the second or foreign language, L2, which is learned later in childhood or as an adult. The competence, or grammar, of the native speaker is taken to be the ultimate reference for linguistic description.
Bilingual speakers have two L1s, and the third or fourth languages of multilingual speakers may also be referred to as second languages (L2).
The field takes second language as its umbrella term, where necessary distinguishing foreign languages, which are learned in contexts where the target language is not commonly spoken (e.g., learning English in France), from second languages, learned in areas where the target is the ambient language (e.g., learning French in France).
Finally, L2 can refer to both the learner's developing interlanguage or the native speaker's competence in that language; the term target language is used when it is necessary to disambiguate.
Modern second language research is often dated to Corder's seminal 1967 paper on learner errors.
Read more: Learner errors