Axis of rotation
In geometry, the axis of rotation of a rotating body is a line such that for every point of the body its distance to the line remains constant under the rotation, and the point remains in the same plane perpendicular to the axis. Thus the point moves in a circle in that plane.
A more general case is that of a screw axis, where rotation is accompanied by translation along the axis.
See also rotational symmetry.da:Akse