Any time you take an examination, you are likely to encounter questions that require definitions. You might, for example, be asked to define behaviorism; tell what a cell is; explain the meaning of the literary term naturalism; or define authority. Such exam questions cannot always be answered in one or two sentences. In fact, the definitions they call for often require several paragraphs.  Most people think of definition in terms of dictionaries, which give brief, succinct explanations of what words mean. But definition has much wider application. It also includes explaining what something, or even someone, is-that is, its essential nature. Sometimes a definition can be given in a sentence. At other times it requires a paragraph, an essay, or even a whole book. These longer, more complex definitions are called extended definitions.

Extended definitions are useful for many academic assignments besides exams.  A thoughtful definition can clarify precise terms as well as more general concepts central to any academic discipline. Definitions can explain abstractions like freedom or controversial terms like right to life or slang terms whose meanings may vary from locale to locale or change as time passes. In a given writing situation, a definition can be essential because a term has more than one meaning, because you are using it in an uncommon way, or because you suspect the term is unfamiliar to your readers.

Many extended-definition essays include shorter definitions like those in the dictionary. Moreover, essays with other dominant patterns of development often incorporate brief definitions of terms to clarify points or establish basic information that equips the reader to understand the rest of the discussion.  Whether it appears in another kind of essay or acts as a center for an extended definition, the brief formal (or dictionary) definition establishes the basic meaning of a term. It presents the term, its general class, and the qualities that differentiate it from other terms in the same class:

Term                      Class                         Differentiation
Behaviorism           is a theory                that regards the objective facts of a
                                                               subject's actions as the only valid basis
                                                                for psychological study.

A cell                     is a unit of                with a nucleus, cytoplasm, and an
                              protoplasm                enclosing membrane.

Naturalism            is a literary                whose original adherents believed that
                              movement                 writers should treat life with scientific
                                                                objectivity.

Authority             is the power                to command and require obedience.

Supplying a dictionary definition of each term you use is seldom necessary or desirable. Readers generally will either know what a word means or be able to look it up easily. Frequently, however, defining your terms is essential--for example, when a word has several meanings, each of which might fit your context; when you are writing about an abstract concept; or when you want to use a word in a special way.

Taken from Patterns for College Writing. 6th Ed. Ed. Laurie G. Kirszner and Stephen R. Mandell. New York: St. Martin's, 1995. 487-489.