Virtual displacement is a concept created to explain the virtual work, which in turn gives the criteria for a system to be at equilibrium. This methodology is adopted to find the trajectory of a contraintless system,by method of Lagrangian dynamics.

Virtual displacement is a displacement by the particle,without involving time. Which is generally considered as the mathematical concept we place for the 'change In configuration' of the system. But how do a system change its configuration in no time?