A solid angle is the area cut out on a unit two-sphere by a cone of directions. Its SI unit is the steradian, and a full sphere subtends steradians.
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A solid angle is a measure of how large an object appears to an observer from a particular point of view, and it indicates the two-dimensional angle in three-dimensional space. Solid angles are measured in steradians (sr), where one steradian corresponds to the solid angle subtended at the center of a sphere by an area on its surface equal to the square of the sphere's radius.