It is not possible to define all terms. Something must remain undefined. Whenever a term is defined, other terms must be used. For example, if a square is defined as a rectangle with equal adjacent sides, what is a rectangle and what are equal adjacent sides? When they are defined, they must be defined in terms of other items. If we try to define everything, the process is endless. When you first studied sets, many terms were defined, but “set” itself remained undefined. Since in geometry we will be dealing with sets of points, sets of lines, and sets of planes, we list here for reference some of the most frequently used definitions.