A radian is a unit of angular measurement used in mathematics and engineering. It is defined as the angle formed when the radius of a circle is wrapped along its circumference. One complete revolution around a circle corresponds to 2\pi radians, which is approximately 6.28 radians. Radian is often used in trigonometry, where it simplifies calculations involving angles. For example, the sine and cosine functions are typically expressed in radians. This unit is particularly useful in calculus and physics, especially when dealing with periodic functions and rotational motion, as it provides a direct relationship between angle and arc length.