A body is said to be in equilibrium when is either at rest or moving with constant velocity with respect to some frame of reference (does it need to be an inertial frame? think about the spacecraft showing up in the Interstellar film). This means the net force acting on the body must be equal to zero.
$$
\sum \vec{F}_{net}=\vec 0
$$
When dealing with extensive bodies, we also need to consider torques $\tau=\vec{r} \times \vec{F}$
$$
\sum \vec{\tau}_{net}=\vec 0
$$
Conservative Forces