Real Dictionary

steradian in American English
the basic unit of solid angular measure in the SI system, equal to the solid angle subtended at the center of a sphere by an area, on its surface, that is equal to the square of its radius: one sphere equals 4π steradians
noun: a solid angle at the center of a sphere subtending a section on the surface equal in area to the square of the radius of the sphere

steradian in British English
noun: an SI unit of solid angle; the angle that, having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to the square of the length of the radius