Comprehensible input is a term used in language acquisition that refers to language input that can be understood by listeners, even if they cant understand all the words or structures in it. It is described as one level above that of the learners if it can only just be understood. The concept comes from American linguist, education researcher, and activist Dr. Stephen Krashen, who developed the input hypothesis, a group of five hypotheses of second-language acquisition in the 1970s and 1980s. According to Krashens theory of language acquisition, giving learners comprehensible input helps them acquire language naturally, rather than learn it consciously.

Comprehensible input is an instructional technique in which teachers provide input that allows English language learners to understand most, but not necessarily all, of the language. Delivering comprehensible input is a lot like scaffolding, except it focuses not only on content knowledge but also on language proficiency in the domains of reading, writing, speaking, and listening.

It is important to note that just any input is not sufficient; the input received must be comprehensible. Learners will crack the speech code only if they receive input that is comprehended at two levels. They must not only understand what is meant but also how things are quite literally expressed, i.e., how the different meaning components are put together to produce the message.

In summary, comprehensible input is language input that can be understood by listeners despite them not understanding all the words and structures in it. It is a crucial and necessary ingredient for the acquisition of language, according to Krashens input hypothesis.