A unit of measure of solid angles. Formally, it is the angle subtended at the center of the sphere by a portion of the surface whose area is equal to the square of the radius of the sphere. There are 4 pi steradians in a sphere.
the standard unit of solid angle measure in mathematics. Just as there are 2 pi radians in a circle, there are 4 pi steradians in a sphere. Thus one steradian equals about 0.079 577 sphere. There are 129 600/ pi = 41 252.96 square degrees in a sphere, so 1 steradian also equals about 3282.806 square degrees. The unit originated in the 1870s by analogy with the radian.
(str) A solid angle with vertex at the center of a sphere of radius r that encompasses an area of r2 on the surface of the sphere.