Definition

uses words to fix the meaning of a thing -- to make it "definite". The short definition (a paragraph or a single sentence) is essential to technical writing. For instance, the Mechanism Description and the Process Description each begin with a call for a definition. A definition answers the question "what is it?" Good definitions employ the following formula:

species = genus + differentia thing to be defined = group to which
the thing belongs + specific details that separate it
from other things in its group A batten is a tapered piece of wood that fits into a pocket in the trailing edge
of a sail, helping it hold the shape
that allows it to propel a boat.

Sometimes definitions might be much longer than one sentence, in which case you are still trying to answer "what is it?" but will be using most other patterns to help answer it: you can define by describing, classifying, comparing, etc.