  The confusion and ambiguity encountered by students, in understanding virtual
displacement and virtual work, is addressed in this article. A definition of
virtual displacement is presented that allows one to express them explicitly
for both time independent and time dependent constraints. It is observed that
for time independent constraints the virtual displacements are the
displacements allowed by the constraints. However this is not so for a general
time dependent case. For simple physical systems, it is shown that, the work
done on virtual displacements by the constraint forces is zero in both the
situations. For allowed displacements however, this is not always true. It is
also demonstrated that when constraint forces do zero work on virtual
displacement, as defined here, we have a solvable mechanical problem. We
identify this special class of constraints, physically realized and solvable,
as the ideal constraints. The concept of virtual displacement and the principle
of zero virtual work by constraint forces are central to both Lagrange's method
of undetermined multipliers, and Lagrange's equations in generalized
coordinates.
