noun
plane angle
A mathematical term for an angle formed by two lines in a plane, as opposed to a solid angle.
平面角は通常ラジアンで表される。
Plane angles are usually expressed in radians.
立体角 (solid angle) is the three-dimensional counterpart of 平面角, measured in steradians.
Compound of 平面 (plane) and 角 (angle).