In the context of teaching a second language, input refers to what a learner hears and processes in the target language. The importance of input in the language classroom and as an important part of language acquisition was developed and popularized by a linguist called Stephen Krashen. Krashen’s input hypothesis is a key part of a theory of second language acquisition that also includes five other hypotheses. According to Krashen, input is a necessary part of second language acquisition, but only if used correctly.