Definitions may seem marginal to the sociological enterprise but can be very useful; however, they can also lead to serious errors. Examples of both are given in this article. Different types of definitions are presented, and their relevance for sociology is highlighted. A stipulative definition, for example, is very useful in sociology, as opposed to lexical and ostensive definitions. The definition of a concept that is used in a sociological analysis has to be sociological in nature, and the concept cannot (...) have the same meaning as it has in everyday language. Additional items to bear in mind when a definition is put together are discussed, including some tips on how to proceed that come from political science (Sartori, Collier, Goertz). A definition can also be very helpful in the research process itself by indicating which are the important aspects of some phenomenon to focus on (Weber, Blumer). On the negative side, a term that is not properly defined may lead to misunderstandings as well as to a false sense of agreement with other uses of the term. (shrink)