Principle of Virtual Work

An equilibrium statement that the total virtual work done by external and constraint forces vanishes for every admissible virtual displacement consistent with the constraints; used to derive equilibrium equations without explicitly computing reactions.

For a mechanical system in equilibrium, the total virtual work done by internal and external forces for any kinematically admissible virtual displacement is zero; virtual displacements are infinitesimal, kinematically consistent variations of the actual configuration, not actual motions.