Words are labels, or symbols, for things and ideas, called their referents. A geologist would immediately know the meaning of such words as batholith, geode, phenocryst, and scoria, for he would be familiar with their referents. The educated layman, however, might be left in the dark if these words were not defined for him.

As always, let your audience be your guide in the matter of definition. In general, three types of situations call for the definition of words:

I. Words That Need to be Defined.

A. Technical words used for familiar things: If no familiar word exists, define the technical word in terms of the familiar thing--hydroponics (growing plants with out soil in a chemical solution); analgesic (painkiller).

B. Familiar words used in a technical sense: puddle, float, apron, backlash, garbage, cheater, christmas tree, diaper, cracker, dirty, dwell, grandfather, lake, tipple.

C. Technical words used for unfamiliar things: dielectric, parity bit, gimbal, ctenidium, siderite.

II. Methods of Definition.

A. Informal: Used when the word or term, and not the referent, is unfamiliar to the reader. These definitions are single word or phase substitutions. While not complete definitions, they are particularly adapted for use in the text of a discussion (as in I.A.).

B. Formal: A logical definition consisting of three parts, the species, the genus, and the differentia. The species is the term to be defined; the genus is the class to which the species belongs; the differentia is the list of specific qualities that distinguishes the term to be defined from all other species in that genus. Examples: Brazing is a welding process wherein the filler metal is a nonferrous metal or alloy whose melting point is higher than 1000 F but lower than that of the metal or alloys to be joined. A micrometer caliper is a C-shaped gauge in which the gap between the measuring faces is minutely adjustable by means of a screw whose end forms one face.

III. Cautions:

A. Do not repeat the word to be defined in the genus or differentia.

B. Do not let a single instance or example stand as a definition. (Avoid the "is when," and "is what" phrases).

C. Do not use other unfamiliar terms in the genus or defferentia.

IV. Extended Definition: An extended definition begins with a formal definition and amplifies it in several different ways.

A. Give further explanation and more detail.

B. Give specific, concrete examples.

C. Compare the unfamiliar term to something the reader would probably be familiar with.

D. Give the derivation of the word.

E. Give a physical description of the thing defined.

F. Give a description of the process involved.

G. Give an explanation of the basic principle involved.

H. Discuss the causes or effects of the term being defined.

I. Tell what the term is not, or differentiate it from closely related terms.