二面角指两个半平面所夹的空间部分。这两个半平面所交的直线叫做二面角的棱，这两个半平面叫做这个二面角的面。

A dihedral angle is the angle between two intersecting planes. In chemistry it is the angle between planes through two sets of three atoms, having two atoms in common. In solid geometry it is defined as the union of a line and two half-planes that have this line as a common edge. In higher dimension, a dihedral angle represents the angle between two hyperplanes.