Definition
A definition is a statement that fixes the meaning of a term, either by stipulation (declaring how the term will be used in a particular context) or by report (describing how the term is used in ordinary language). Logicians distinguish several kinds. An intensional definition specifies the necessary and sufficient conditions a thing must satisfy to fall under the term — "a triangle is a closed plane figure with exactly three straight sides." An extensional definition lists or characterizes the things to which the term applies. An ostensive definition points to examples. An operational definition specifies a procedure for determining whether the term applies.
A useful definition is neither too broad (it must exclude things that are not instances) nor too narrow (it must include all instances), and it must avoid circularity (the defined term cannot appear in the defining phrase).
Why it matters
How it works
Producing a good intensional definition typically follows a two-step structure inherited from Aristotle: identify the genus (the broader class to which the thing belongs) and then specify the differentia (the feature that distinguishes this kind from other members of that class). "A human is an animal (genus) that is rational (differentia)." This genus-plus-differentia pattern produces definitions that are both informative and testable: each clause can be examined separately, and counter-examples can target either the broader category or the distinguishing feature.
Definitions are not merely descriptive; they are normative tools that shape inference. A debate over whether a particular act constitutes "free speech" or whether a particular policy is "socialism" is almost always a debate about which definition to adopt. Sophisticated arguers acknowledge this openly and propose a working definition before drawing conclusions; less careful arguers rely on the shifting meanings their audience supplies, which is the entry point for the fallacy of equivocation.