STERADIAN

unit of solid-angle measure in the International System, defined as the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere's radius. Since the complete area of a sphere is 4p times the square of its radius, the total solid angle about a point is equal to 4p steradians. Derived from the Greek for solid and the English word radian, a steradian is, in effect, a solid radian; the radian is an International System unit of plane-angle measurement defined as the angle of a circle subtended by an arc equal in length to the circle's radius.