In the mechanics, a body is in (mechanical) equilibrium, if it is either at rest or moves in a straight line with constant velocity and is not in an accelerated rotational motion.  This means that the vector sum of all forces and the vector sum of all torques acting on the body vanish.

A system is in thermodynamic equilibrium, if its observable (macroscopic) properties do not alter with time.  This contains

• the thermal equilibrium, i.e. the temperature is the same in every point;
• the chemical equilibrium, i.e. the consistency does not alter;
• the mechanical equilibrium, i.e. there is no macroscopic motion within the system.