Provide definitions for any terms that have meanings other than their normal dictionary definitions or the meanings that courts have given them. Questions about the terminology used in a patent application usually arise when the term is used in a claim. Defining terms can be a helpful way to eliminate the need to repeat information in the body of the application (called the specification) and/or the claims.
Warning! When you define a term used in a patent application, the term means only that. This can be a disadvantage if the term would have otherwise meant something more favorable to you (to the hypothetical "person with ordinary skill in the art") in the absence of the definition.