 steradian

/steuh ray"dee euhn/, n. Geom.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. Abbr.: sr[188085; STE(REO) + RADIAN]
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▪ unit of measurementunit of solidangle measure in the International System of Units (SI), defined as the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere's radius. Since the complete surface area of a sphere is 4π times the square of its radius, the total solid angle about a point is equal to 4π steradians. Derived from the Greek for solid and the English word radian, a steradian is, in effect, a solid radian; the radian is an SI unit of planeangle measurement defined as the angle of a circle subtended by an arc equal in length to the circle's radius.* * *
Universalium. 2010.