$(a)$ Define acceleration.
$(b)$ $A$ stone describes a circular path with a constant speed. State the type of motion of the stone.

(N/A) Acceleration is defined as the rate of change of velocity of an object with respect to time. Mathematically, it is the ratio of the change in velocity to the time taken for that change: $a = \frac{v - u}{t}$.
$(b)$ When a stone moves in a circular path with a constant speed, its direction of motion changes at every point. Since velocity is a vector quantity (speed with direction), the change in direction implies a change in velocity. This type of motion is known as $\text{Uniform circular motion}$.