We define terms in order to make their meaning understood, of course. But we might ask, what does understanding the meaning of a term give us? Let’s dig a little deeper into this question.

There are at least six purposes for defining terms.

1. To show relationships between terms. A good example is Aristotle, in his Rhetoric, defining indignation as “a feeling of pain caused by the sight of undeserved good fortune” and pity as “a feeling of pain cause by the sight of undeserved bad fortune.” This helps us to see how these terms are related, and that a just man would tend to feel both indignation and pity, because he has an understanding of what is deserved.
2. To remove ambiguity. An ambiguous word has more than one meaning, like the word bar. “A tall man walked into a bar. He said ‘ouch!'” We define terms in order to deal with equivocations like this, making it clear which meaning we have in mind when we use the word.
3. To reduce vagueness. A vague word is unclear as to its extent. The instructions for constructing a model, instead of saying “For adults,” will give a more precise definition for adult, e.g. “For people 18-years old and up.” Such definitions will usually not be found in dictionaries,  applying only to particular situations.
4. To increase vocabulary. Much teaching consists in giving students new meanings for new terms, thus enlarging their vocabulary and adding to their storehouse of knowledge. Thus, when we learn the definitions of ambiguous and vague, our personal vocabulary has increased, and we can begin to understand how these words relate.
5. To explain theoretically. Often a definition will be theoretical in a way that accepting the definition implies that one is buying into a particular theory. If you agreed that light is “visible electromagnetic waves,” this is the same as accepting the wave theory of light (as opposed to the particle theory).
6. To influence attitudes.  Sometimes we give a definition for a term in order to make people feel a certain way, either good or bad, about the thing being defined. When you father calls the television “a one-eyed brain sucker,” he is trying to influence your attitude about watching it.

There are no doubt additional purposes for defining terms, but this list of six perhaps goes a little ways toward expanding our understanding.