What is changing velocity ?

Hint:Acceleration is nothing but the rate of change of velocity. Commonly, acceleration means the speed of the particle is changing, but not always. When a body moves in a circular path at a constant speed, the direction of its velocity is changing, that is why it is still accelerating.

Complete answer:
The changing velocity is termed as Acceleration. Acceleration is defined as the rate of change of the velocity of an object with respect to time. Acceleration is a vector quantity (they have both magnitude and direction). The direction of an object's acceleration is given by the direction of the net force acting on that object.

The magnitude of the acceleration of an object, as defined by Newton's Second Law, is the combined effect of two causes:
-The net balance of all external forces acting on that object — magnitude is directly proportional to this net resulting force;
-That object's mass, depending on the materials out of which it is made — magnitude is inversely proportional to the object's mass.

The SI unit of acceleration is \[m{s^{ - 2}}\] or \[m/{s^2}\]. Since acceleration is defined as the rate of change of velocity of an object. So the acceleration is given by the formula
\[a = \dfrac{{{v_2} - {v_1}}}{t}\]
Where \[a\] the change in velocity or acceleration is, \[{v_2},{v_1}\] are the velocities of the body at time interval \[t\].
If there is no change in velocity of the body during the time interval then the acceleration is said to be zero acceleration.

Note: When a body rotates in a circular path with a constant speed, it is still accelerating, since the direction of its velocity is changing at every instant. The acceleration due to gravity of a body under free fall is constant and is equal to \[9.8\,m{s^{ - 2}}\].