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 radiusSymbol: sr

A unitless measure of solid angles. A solid angle projecting from the center of a sphere and cutting its surface has a measure ofsr2 steradians, where s is the surface area of the sphere cut out by the solid angle, and r is the radius of the sphere. See also radian.