Point, line, and plane, together with set, are the undefined terms that provide the starting place for geometry. When we define words, we ordinarily use simpler words, and these simpler words are in turn defined using yet simpler words. This process must eventually terminate; at some stage, the definition must use a word whose meaning is accepted as intuitively clear. Because that meaning is accepted without definition, we refer to these words as undefined terms. These terms will be used in defining other terms. Although these terms are not formally defined, a brief intuitive discussion is needed.
A point is the most fundamental object in geometry. It is represented by a dot and named by a capital letter. A point represents position only; it has zero size (that is, zero length, zero width, and zero height). Figure 1 illustrates point C, point M, and point Q.
A line (straight line) can be thought of as a connected set of infinitely many points. It extends infinitely far in two opposite directions. A line has infinite length, zero width, and zero height. Any two points on the line name it. The symbol ↔ written on top of two letters is used to denote that line. A line may also be named by one small letter (Figure 2).
Points that lie on the same line are called collinear points. If there is no line on which all of the points lie, then they are noncollinear points. In Figure 3 , points M, A, and N are collinear, and points T, I, and C are noncollinear.
Figure 3 Three collinear points and three noncollinear points.
A plane may be considered as an infinite set of points forming a connected flat surface extending infinitely far in all directions. A plane has infinite length, infinite width, and zero height (or thickness). It is usually represented in drawings by a four‐sided figure. A single capital letter is used to denote a plane. The word plane is written with the letter so as not to be confused with a point (Figure 4 ).
Figure 4 Two planes.