A side is one of several kinds of elements that form a geometric boundary. A side of an angle is either of the two rays having their common endpoint at the vertex. A side of a polygon is one of the line segments joining consecutive vertices and forming its boundary. More specifically, it is a polygon edge. For a polyhedron, the word "side" is sometimes used informally for a face, although "face" is the more precise term.
For a directed angle, the initial side is the ray from which the rotation begins and the terminal side is the ray at which it ends.
