A shot, usually a closeup of some detail, or landscape, that is used break up a matching action sequence, and is often very helpful in editing to rescue you from an impossible break in continuity or coverage. A cutaway, as the name implies, is a shot that does not focus on some detail of the shot before or after it but cuts away from the action at hand, unlike an insert shot. However, the two terms are sometimes used vaguely or interchangeably, although this is not always a useful practice. The best cutaways are the ones that have some logic to them, that relate to the scene.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission