A figure formed by two line segments that extend from a common point. The term angle is also often used to refer to the measure of an angle, most commonly in units of degrees or radians. Generally, the measure of an angle is the amount by which one of the lines must be rotated counterclockwise about the common point to coincide with the other line.

Figure 1

The angle between two intersecting planes is measured as the angle between two intersecting lines, one lying in each plane and both perpendicular to the common line.