The angle between two lines (or line segments) that have a point in common is the amount of rotation that separates the two lines. Note that the lines must have a point in common in order to define an angle between them. Angles were described by Euclid as "the inclination to one another of two lines which meet each other in the same plane, and do not lie straight with respect to each other"
There are several units used to measure angles, the most common are:
Degree: Most commonly used in everyday life. It is defined as 1/360 of a full circle, therefore a circle has 360 degrees.
Radian: Most commonly used in science and engineering, is the angle subtended by an arc of a circle which have the same length as the circle radius. One full circle is equal to 2π radians, and one radian is 180/π degrees.
A simple way to classify angles is the following: