Answer:
Radian Measure
Definition
Radian measure is the ratio of the length of a circular arc (a) to the radius of the arc (r).
Since radian measure is the ratio of a length to a length, the result is a pure number that needs no unit symbol. Most mathematicians do not write a unit attached to this ratio, although some people add the unit "radians" (symbol: "rad"), to indicate an angular measure, or length along a unit circle.