Rules of Definition

The following rules have been traditionally laid down by logician;

1. The Definition should be coextensive with the thing defined. The Definition should neither be broader than the definiendum so as to include any other object, nor should it be narrower so as to exclude parts of the defined object. This is what classical scholars mean when they refer to a Definition as a "comprehensive but exclusive term".

2. A Definition should state the essential attributes of the species.

3. A Definition must not be circular. That is, it should not define by synonyms and antonyms, e.g. defining a parent as the person who has a child.

4. A Definition must not be expressed in ambiguous, obscure, or metaphorical terms.

5. A Definition should not be negative where it can be affirmative.

Source: Irving M. Copi, Introduction to Logic.