The radian is the standard unit of angular measure, used in many areas of mathematics. An angle's measurement in radians is numerically equal to the length of a corresponding arc of a unit circle, so one radian is just under 57.3 degrees (when the arc length is equal to the radius); a full circle corresponds to an angle of 2π radians. In the SI, the radian has the symbol "rad"; it was a supplementary unit until that category was abolished in 1995, and is now considered a derived unit.Diagram: Lucas V. Barbosa