Language attrition can be defi ned as a reduction, weakening or loss of a fi rst, second, third, or more language in an individual person or in a community. The focus of research in this area is on language attrition in individuals, even though the defi nition of the term also includes the attrition of a language within a community (i.e., language death, when languages with few speakers gradually disappear). From typological as well as, for example, psycholinguistic or sociolinguistic perspectives, interesting parallels can be found between attrition of a language in an individual and attrition of a language in the whole community. For theoretical and applied linguistics, both are of equal interest, but this entry focuses on attrition in the individual.Some researchers defi ne attrition in the individual more narrowly. According to Köpke and Schmid, attrition is "the non-pathological decrease in a language that had previously been acquired by an individual" (2004, p. 5). The community or research area of language attrition has its origin in the 1980 conference "Loss of Language Skills" (Lambert & Freed, 1982). By including the word "non-pathological" in the defi nition, cases of L1 loss in an L1 environment due to aphasia, dementia, and so on are excluded. Interestingly enough, cases of L1 loss by normal aging are not excluded by the inclusion of this term. They are, however, not usually studied within this area and will, therefore, also be excluded here.Even the narrow defi nition, however, retains the problem of what should be classifi ed as "attrition" of an acquired L1, L2, and so on. Given that the term "non-pathological" has excluded some attrition phenomena, with, for example, neurological causal factors, the following question arises: How should we regard "attrition"-should we regard it as "normal" or "abnormal" language deterioration? Where is it reasonable to draw the line between normal infl uence between languages in a bilingual or polyglot and a state or process that can be called attrition of one or more languages? This question has not been solved and it is perhaps reasonable not to try to draw a defi nite line, but to consider these phenomena as varying more gradually (see Schmid & Köpke, 2007).Language attrition can occur for many reasons. A monolingual person can lose his or her fi rst and only language, a bilingual person can lose one or both of his or her two languages, and a multilingual person can lose one or more of his or her languages. This can take place in an environment where one or more of these languages are used to varying degrees. The degree of attrition can vary and different parts or aspects of language can be affected.There are many possible conditions where language loss can occur. According to a taxonomy suggested by van Els (1986), L1 can be lost in an L1 environment, in cases of aphasia and dementia. This can also be studied in normal aging. The L1 can also be lost in an L2 environment (minority communities, migrants). The L2 can be lost in an L1 environment (e.g., by stude...