This essay first outlines a philosophical theory of concepts and then applies it to two areas of relevance to psychiatrists, especially forensic psychiatrists. In the philosophical theory, the respective roles of verbal and non-verbal definitions are illuminated, and the importance of the phenomenon of division of semantic labour is stressed. It is pointed out that vagueness and ambiguity of a term often result when the term is used for several practical purposes at the same time. Such multi-purpose uses of terms may explain both the current problems associated with the Swedish forensic-psychiatric concept of a severe mental disorder and some of the shortcomings of DSM-IV. (C) 2009 Elsevier Ltd. All rights reserved.