We know that the angular acceleration is the rate of change of angular velocity with the respect to time. It is a vector quantity and may be represented by drawing a vector diagram with the help of right hand screw rule. Consider a disc as shown in below fig:

Revolving or spinning about the axis OX known as axis of spin in anticlockwise when seen from the front with an angular velocity ω in a plane at right angles to the paper.

After a short interval of time δt. Let the disc be spinning about the new axis of spin OX’ with an angular velocity. Using the right hand screw rule, the initial angular velocity of the disc (ω) is represented by vector ox. The final angular velocity of the disc (ω+δω) is represented by vector ox’ shown in fig:

Where dθ/dt is the angular velocity of the axis of spin about a certain axis, which is perpendicular to the plane in which the axis of spin is going to rotate. The angular velocity of the axis of spin (dθ/dt) is known as **angular velocity of precession**. It is denoted by ω_{p}. The axis about which the axis of spin is to turn is known as **axis of precession**. The angular motion of the axis of spin about the axis of precession is known as **Precessional Angular Motion**.