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The SI unit of solid angle, equal to the angle at the centre of a sphere subtended by a part of the surface equal in area to the square of the radius.
‘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.’
‘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.’
‘There are also two supplementary units, the radian, and the steradian.’
Origin
Late 19th century: from Greek stereos ‘solid’ + radian.