radian

A radian is a unit used to measure angles. It is defined as the angle at the center of a circle that is formed when the arc length of the circle equals the radius. One radian is approximately equal to 57.3 degrees.1

In mathematics and physics, radians are commonly used for angles, especially in calculus and trigonometry. For instance, trigonometric functions like sine and cosine can be defined using angles measured in radians. This is particularly useful in calculus, where formulas involving these functions are simpler when expressed in radians. Angular velocity, which measures how fast an object rotates, is also frequently expressed in radians per second.23