Definition

A rotation R about a point O through an angle θ is a transformation of the plane onto itself which carries every point P of the plane into the point P′ of the plane such that OP = OP′ and m (∠POP′) = θ. O is called the centre of rotation and θ is called the angle of rotation.

A rotation by angle θ is in the counter clockwise direction, if θ > 0 and in the clockwise direction if θ < 0.