Equilibrium of a body 

Equilibrium, in physics, the condition of a system when neither its state of motion nor its internal energy state tends to change with time.
A simple mechanical body is said to be in equilibrium if it experiences neither linear acceleration nor angular acceleration; unless it is disturbed by an outside force, it will continue in that condition indefinitely.
For a single particle, equilibrium arises if the vector sum of all forces acting upon the particle is zero.