The time rate of change of angular velocity of rotating body.

It is denoted by $\alpha$ and it is a vector.

Unit of angular acceleration is radian/sec $^{2}$ and its dimensional formula is $\left[\mathrm{M}^{0} \mathrm{~L}^{0} \mathrm{~T}^{-2}\right]$

If axis of rotation is fixed, the direction of $\vec{\omega}$ and $\vec{\alpha}$ is also fixed. In this situation, angular acceleration is considered as scalar

$\therefore \alpha=\frac{d \omega}{d t}$ but $\omega=\frac{d \theta}{d t}$,

$\alpha=\frac{d^{2} \theta}{d t^{2}}$