Define angular acceleration.

Angular acceleration is defined as the time rate of change of angular velocity of a rotating body.
It is denoted by the symbol $\alpha$ and is a vector quantity.
The $SI$ unit of angular acceleration is $\text{rad/s}^2$ and its dimensional formula is $[M^0 L^0 T^{-2}]$.
If the axis of rotation is fixed,the direction of the angular velocity vector $\vec{\omega}$ and the angular acceleration vector $\vec{\alpha}$ remains constant. In this specific situation,angular acceleration can be treated as a scalar quantity.
Mathematically,$\alpha = \frac{d\omega}{dt}$. Since angular velocity $\omega = \frac{d\theta}{dt}$,we can express angular acceleration as the second derivative of angular displacement:
$\alpha = \frac{d^2\theta}{dt^2}$.