steradian

steradian

[stuh-rey-dee-uhn]
noun Geometry.
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. Abbreviation: sr

Origin:
1880–85; ste(reo)- + radian

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World English Dictionary
steradian (stəˈreɪdɪən)
 
n
sr 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
 
[C19: from stereo- + radian]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
steradian   (stĭ-rā'dē-ən)  Pronunciation Key 
A unitless measure of solid angles. A solid angle projecting from the center of a sphere and cutting its surface has a measure of s/r2 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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

steradian

unit of solid-angle 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 4pi times the square of its radius, the total solid angle about a point is equal to 4pi 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 plane-angle measurement defined as the angle of a circle subtended by an arc equal in length to the circle's radius

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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