A radian is a unit of measurement for angles, denoted by the symbol "rad" or "c" in the exponent. It is the standard unit of angular measure used in many areas of mathematics and is defined such that one radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius. More generally, the magnitude in radians of a subtended angle is equal to the ratio of the arc length to the radius of the circle. One complete circle is equal to 2π radians, or approximately 6.283 radians. Radians are preferred by mathematicians because they are based on the pure idea of "the radius being laid along the circumference," which often gives simple and natural results when used in mathematics.

To convert radians to degrees, we multiply the angle by 180/π. For example, 3π/2 = 3π/2 × 180/π = 270°. Radians are used heavily in physics and engineering, and they are also used in calculus and most other branches of mathematics.