In information processing models of learning, output refers to data delivered by the machine after processing; for language acquisition, output refers to spoken or written language samples produced by the learner, providing evidence about the learner's internal grammar of the language (interlanguage).
Early cognitive theories of second language acquisition, both generativist accounts and Krashen's input hypothesis, laid great emphasis on the role of input in acquisition, neglecting the importance of learner production, or output in the acquisition process.
Swain formulated the output hypothesis to capture her view that second language acquisition requires output, or learner production, if it is to proceed efficiently. This hypothesis represents a constructivist view of interlanguage development, depending on an active learner constructing and testing hypotheses in using the target language.
Working in an input-interaction framework, Gass assigns a number of reasons why output is necessary to learners: output provides learners with opportunities for grammatical processing, feedback, hypothesis-testing, and developing automaticity (Gass, 2003).