a quantity that characterizes the rate of rotation of a rigid body. If a body rotates uniformly about a fixed axis, its angular speed is ω = Δφ/Δt, where Δφ is the increment of the rotation angle φ in the time interval Δt. In general, angular speed is the magnitude of the vector quantity angular velocity and is equal to the ratio of the elemental rotation angle dφ and the corresponding elemental time interval dt—that is, ω = dφ/dt. The angular velocity vector is aligned with the rotation axis in the direction from which the rotation of the body is seen as counter-clockwise (in a right-handed coordinate system). Angular speed has the dimension T–1, where T is time. In English the term “angular velocity” is sometimes used synonymously with “angular speed.”