Average velocity is the total change in displacement divided by the time taken. It can only be defined over a time interval, so takes a finite interval to measure. E.g. if a ball travels $(3 \mathbf i + 2 \mathbf j)m$ in 2s, its average velocity is $(1.5 \mathbf i + \mathbf j)ms^{-1}$.

Instantaneous velocity is the derivative of displacement with respect to time, so is the limit of the average velocity as the time taken goes to zero. E.g. if the displacement is given by $\mathbf x (t) = \frac{1}{2} \mathbf a t^2 $ then the velocity is $\mathbf v (t) = \mathbf a t$. It is defined at every point in time, and theoetically requires no time interval to measure (although it would obviously take time to measure it this is related to the use of measuring instruments, not the definition of velocity)

Average velocity is the total change in displacement divided by the time taken. It can only be defined over a time interval, so takes a finite interval to measure. E.g. if a ball travels $(3 \mathbf i + 2 \mathbf j)m$ in 2s, its average velocity is $(1.5 \mathbf i + \mathbf j)ms^{-1}$.

Instantaneous velocity is the derivative of displacement with respect to time, so is the limit of the average velocity as the time taken goes to zero. E.g. if the displacement is given by $\mathbf x (t) = \frac{1}{2} \mathbf a t^2 $ then the velocity is $\mathbf v (t) = \mathbf a t$. It is defined at every point in time, and theoretically requires no time interval to measure (although it would obviously take time to measure it this is related to the use of measuring instruments, not the definition of velocity)