An ELT Glossary : The Affective Filter Hypothesis
Definition : One of the five hypotheses in the Monitor Model proposed by Krashen in his work in the seventies and eighties.
Krashen proposed the idea of an "affective filter" in the brain. Negative affective factors (by "affect" we mean emotions and psychological states like motivation, depression, fear,self-esteem, enthusiasm etc) would cause the "filter " to be raised and to block learning/acquisition from taking place, while positive affective factors would cause it to be lowered, facilitating intake.
Krashen was not the first person to link affective factors to the effectiveness of learning. He drew directly on the work of Dulay and Burt (1977), but the idea (if not the name - a metaphor for a process which has no known "concrete" neurological mechanism) is common in all writers on humanistic education, whether general education (eg Rogers) or within the humanistic movement in EFL - which was in full swing when Krashen started writing.
Reference : Dulay, H. and M. Burt, “Remarks on creativity in language acquisition,” in M. Burt, H. Dulay and M. Finnochiaro (eds), Viewpoints on English as a Second Language, Regents, New York, pp. 95-126, 1977
Further reading : Baker, C. and Prys Jones, S. Encyclopedia of Bilingualism and Second Language Acquisition