An instantaneous centre of rotation is defined as the point in a body undergoing planar movement that has a zero velocity, and each and every point on a body rotates about that point, at a given instant.

Consider the following member of a link:

-At a certain instant, point A moves with velocity V_A and point B moves with velocity $V_B.$

-Point I is such that, both the velocities are perpendicular to the line joining I and their corresponding points.

-We know that, in case of a circular motion, velocities are tangential to the circle of rotation and perpendicular to the radius.

-Similarly, in this case, both the points, A and B, appear to move about point I. Thus, point I is the instantaneous centre of rotation.

-At the next instant, the I-centre for the member AB may change.