Definitions orient readers. Definitions help readers place new concepts in context. Definitions explain new terms and concepts to readers. The traditional way to define is to put the term in a class and then explain how it is different from other members of the class:

"A camera is a device (the class) for taking photographs (the difference)."

Often, however, writers use an extended definition because the reader needs to understand the concept, not just the term. In an extended definition, use one or more strategies to make the term familiar to your audience; for example, compare or contrast, use a common example, explain cause and effect, or add a visual aid.

The following paragraph explains cause (within 300 light-years), uses a common example (spherical ball), and a comparison (mothballs):

"In other places in the sky, thousands or hundreds of thousands of stars of a common origin may be located within 300 light-years or so, forming a huge spherical ball. These groupings are called globular clusters. In the northern sky, the globular cluster M13 in the constellation Hercules is the easiest to see. A globular cluster may look like a hazy mothball to the naked eye or when viewed through a small telescope; larger telescopes are necessary to see individual stars in this type of cluster . . ." (Menzel and Pasachoff, p. 116).