当一个质点、刚体或质点系的速度矢量为常矢量时，就称该物体或系统处于力学平衡状态。特别地，当一个静止的质点或质点系处于力学平衡状态时，该质点或质点系即处于静力平衡状态。物体的力学平衡状态在任何和该物体相对静止的参考系中均不变。

当外力对质点系上任何一点所产生的合力矩时，称质点系处于转动平衡状态。

更一般地，根据变分原理，在保守系统中，当位形空间中某势能的泛函（广义位势函数）对广义坐标的梯度在某点为零时，系统在该点处于平衡位置。

In classical mechanics, a particle is in mechanical equilibrium if the net force on that particle is zero. By extension, a physical system made up of many parts is in mechanical equilibrium if the net force on each of its individual parts is zero.

In addition to defining mechanical equilibrium in terms of force, there are many alternative definitions for mechanical equilibrium which are all mathematically equivalent. In terms of momentum, a system is in equilibrium if the momentum of its parts is all constant. In terms of velocity, the system is in equilibrium if velocity is constant. In a rotational mechanical equilibrium the angular momentum of the object is conserved and the net torque is zero. More generally in conservative systems, equilibrium is established at a point in configuration space where the gradient with respect to the generalized coordinates of the potential energy is zero.