Radian

A radian is a unit of measuring angles. It is shown by the symbol "rad"^[1] or, less often, ^c (for circular measure). The radian was once an SI supplementary unit, but was changed to a derived unit in 1995. In a circular sector with a central angle of one radian, the arc length is equal to the length of the radius of the circle it is part of.

1. ↑ "List of Geometry and Trigonometry Symbols". Math Vault. 2020-04-17. Retrieved 2020-08-31.