An angle is formed by two rays sharing a common endpoint called the vertex.
Angles are measured in degrees (°) or radians. A full circle is 360° or 2π radians. Angles are classified as acute (< 90°), right (90°), obtuse (90°-180°), straight (180°), reflex (180°-360°), or full (360°). Angles are fundamental in geometry, trigonometry, navigation, engineering, and physics for describing rotations, directions, and shapes.
The degree measurement system originated from the Babylonians who used a base-60 number system. The division of a circle into 360 degrees likely came from the Babylonian calendar (360 days). Radian measurement, a more natural unit based on the radius of a circle, was introduced by Roger Cotes in the 18th century and later popularized by Leonhard Euler.