Two non-coincident plane angles and in angle standard position are said to be coterminal if the terminal side of is identically the same as the terminal side of .

In general, given a plane angle measured in radians, is coterminal to if and only if for some positive integer . Similarly, if is a plane angle coterminal to a plane angle measured in degrees, then for some positive integer . In the event that , then and are coincident.

In the figure above, the non-coincident angles and are coterminal angles.