Acquisition and Learning
Definition : Acquisition refers to the process of assimilating a language non-consciously through exposure to that language rather than by formal instruction, conscious understanding of rules etc (learning).
Example : A child acquires, rather than learns, their L1.
The Acquisition/Learning Hypothesis
One of the five hypotheses in the Monitor Model proposed by Krashen in his work in the 70s and 80s. Krashen suggested that, in the L2 situation, acquisition and learning were two distinct neurological processes. He also claimed (and this is where the hypothesis becomes most controversial) that only acquired language is available for fluent, spontaneous use and that acquisition cannot be affected by learning - ie "learning" the language will not replace or speed up acquisitional processes. He suggested that the ability to acquire language is not lost at puberty, as advocates of the critical period hypothesis would claim, but remains possible throughout adulthood.
Criticisms : Many writers would argue that, on the contrary, conscious "noticing" and deep cognitive processing is not only an aid to L2 acquisition but also essential for it.
Further reading : Baker, C. and Prys Jones, S. Encyclopedia of Bilingualism and Second Language Acquisition