What is the name of this principle?

If the speed of the centre of mass $\vec{v}_{CM}$ of a solid is constant ($cte$), then the sum of the exterior forces that exerts into this solid $\vec{F}_{ext}$ is zero, and the opposite is true, namely that:

$$\vec{v}_{CM}=cte\Leftrightarrow\sum\vec{F}_{ext}=\vec{0}$$