radian definition
Definition of a radian. A radian is a unit of angular measure. A turn in a full circle has an angle of 2Ï radians. A radian is a unit of measuring angles. It is shown by the symbol rad or, less often, c (for circular measure). The radian was once an SI unit, but was done away with in 1995. A milliradian is 1/1000 of a radian.