The relationship between the notion of "error" in linguistics and language teaching theory and its potential application to error correction in the second language classroom is examined. Definitions of "error" in psycholinguistics, native speech, and English second language instruction are discussed, and the relationship of interlanguage competence and "errors" and "mistakes" is analyzed. It is argued that existing theoretical distinctions between "error" and "mistake" are not being contextualized in appropriate correction in the classroom, and that finer distinctions in categorizing errors could lead to more suitable and effective methods of correction. A system for classifying errors is proposed. The system is based on theory and on data gathered in the English second-language classroom. Four complementary ways of categorizing error are outlined: (1) as error vs. mistake; (2) as a linguistic phenomenon (grammatical, discourse, phonological, or lexical); (3) by gravity of the error (interfering with intelligibility or communication, stigmatizing or irritating, common, or high-frequency); and (4) by difficulty of correction. (Contains 48 references.) (MSE)