The SI unit of solid angle, equal to the angle at the center of a sphere subtended by a part of the surface equal in area to the square of the radius.
- ‘There are also two supplementary units, the radian, and the steradian.’
- ‘A sphere subtends 4 pi square radians (steradians) about the origin.’
- ‘The solid angle in question is only p / 4 steradian, one-sixteenth of the entire sky.’
- ‘This is the power of the light that is emitted per projected surface area into a given solid angle, and it is measured in watts per square metre per steradian.’
Late 19th century: from Greek stereos solid + radian.