What do you mean by the term relative velocity?

(N/A) Relative velocity is the velocity of an object as observed from a specific frame of reference.
Consider two frames of reference,$A$ and $B$,moving with a constant velocity with respect to each other. Let $P$ be a particle.
From the figure,the position vector of particle $P$ with respect to the origin $O$ of frame $A$ is $\vec{r}_{P,A} = \vec{OO'} + \vec{O'P}$.
Since $\vec{OO'} = \vec{r}_{B,A}$ (position of origin $O'$ of frame $B$ w.r.t. origin $O$ of frame $A$) and $\vec{O'P} = \vec{r}_{P,B}$ (position of particle $P$ w.r.t. origin $O'$ of frame $B$),we have:
$\vec{r}_{P,A} = \vec{r}_{P,B} + \vec{r}_{B,A}$
Differentiating this equation with respect to time $t$:
$\frac{d}{dt}(\vec{r}_{P,A}) = \frac{d}{dt}(\vec{r}_{P,B}) + \frac{d}{dt}(\vec{r}_{B,A})$
$\vec{v}_{P,A} = \vec{v}_{P,B} + \vec{v}_{B,A}$
Here,$\vec{v}_{P,A}$ is the velocity of particle $P$ with respect to frame $A$,$\vec{v}_{P,B}$ is the velocity of particle $P$ with respect to frame $B$,and $\vec{v}_{B,A}$ is the velocity of frame $B$ with respect to frame $A$.