Although we talk about the velocity of an object in Uniform Circular Motion as being constant, we really mean

The speed is constant.

Speed is a scalar quantity (magnitude only).

Velocity is a vector quantity (magnitude and direction).

For an object in Uniform Circular Motion the change in velocity is always

directed towards the center of the circle, and this change of velocity

gives rise to the acceleration which is directed towards the center of

rotation. This is called the centripetal acceleration, and it is the centripetal

force (for instance the attraction of the moon towards the earth) which

produces the centripetal acceleration.

The speed is constant.

Explanation:

In reality in circular motion the velocity is never constant because the object keeps changing direction in a continuous fashion. Remember velocity is measured by magnitude and direction. However we use the term constant velocity to mark constant speed for convenience.