The Purpose of Definition

In one line

A definition states what a term means, and it is judged by which of several possible purposes it was made to serve.

In the wording a student can write in an examination: a definition is a statement of the meaning of a term. The term defined is called the definiendum and the words that define it the definiens. A definition is good or bad only in relation to a purpose, and the purposes commonly served are fixing a meaning, settling a dispute, reducing vagueness, introducing a new term, explaining a theory and influencing attitude.

The two halves of every definition

The definiendum is what is being defined: the word or expression whose meaning is in question.

The definiens is what does the defining: the words offered as its meaning.

"A contract is an agreement enforceable by law" has "a contract" as its definiendum and "an agreement enforceable by law" as its definiens. The two are worth naming because the rules at sequence 560 are all rules about the relation between them.

What a definition is about

A definition is about the term, not about the thing. "Water is H two O" looks like a statement about a substance and is a statement about a word. The difference shows in how it is refuted: a definition is not disproved by producing a sample, it is disproved by producing a use of the word that it fails to cover.

A definition states a connotation, and thereby fixes a denotation. This is the pair from sequence 180 and it is the whole machinery of Module IV. The definiens lists attributes; the things possessing them are what the term denotes. Change the attributes and the class changes, by the law of inverse variation.

The purposes

To fix a meaning where it is unsettled. The commonest purpose. A word is used in different senses by different people and the definition picks one.

To settle a verbal dispute. Two people who agree about all the facts and still disagree are often disagreeing about a word. If one says the meeting was "properly convened" because notice went out and the other says it was not because nobody received it, the dispute is about "convened", and a definition ends it. Distinguishing a verbal dispute from a real one is the first thing a definition can do, and it saves an enormous amount of argument.

To reduce vagueness for a particular purpose. A word with a fuzzy boundary is given a sharp one, for one context and no other. This is the précising definition of sequence 570, and it is what every statutory definition clause is.

To introduce a new term. A word that did not previously exist, or a familiar word given an entirely new sense, is brought in by stipulation. "For the purposes of this Act, 'appropriate Government' means..." creates a term of art.