Just as a radian is an angular unit of measure in 2-dimensional space, so a steradian is an angular unit of measure in 3-dimensional space (solid angle).

(or "square radians"). The unit used to quantify a solid angle in three dimensional space. It is analogous to the use of "radian" to quantify an angle in two dimensional space. A sphere subtends 4 pi steradians, a hemisphere subtends 2 pi steradians, etc.