ABSTRACT
Defining can take a variety of forms: for instance, pointing to examples, enumerating all the things that fall under the term at issue, or legislating how a term will be used. One particular type of definition, sometimes called ‘essential’ or ‘real’ definition, has special power as an analytic tool. A real definition of something, X say, would identify a set of properties such that each and every X has all the properties that make up the set and only Xs have that set of properties. A real definition specifies a group of properties each of which is necessary for something’s being an X and which, taken as a group, are also sufficient for something’s being an X. In other words, a definition of X characterizes what all Xs and only Xs have in common. For instance, a widow is a woman who has lost her husband by death and who has not married again. In this case, there are three necessary conditions that together are sufficient for someone’s being a widow.
