In physics, the angular velocity is defined as the rate of change of angular displacement and is a vector quantity. More precisely, it is a pseudo-vector which specifies the angular speed (rotational speed) of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second. It is also measured in other units, such as degrees per hour, degrees per second, etc. The symbol of Angular velocity is usually omega (ω, rarely Ω). The direction of the angular velocity vector is perpendicular to the plane of rotation, in a direction which is usually specified by the right-hand rule.