Radian
Symbol: rad
In geometry and trigonometry, an angle (in full, plane angle) is the figure formed by two rays sharing a common endpoint. Units of angular measurement are defined such that one full circle (i.e. one revolution) is equal to n units, for some whole number n.
The radian is the angle subtended by an arc of a circle that has the same length as the circle's radius. One full circle is 2π = 6.2831854 radians.
The radian is the derived unit of angular measurement in the SI system.