I'll talk here of the principal of virtual work in the context of virtual forces.
The idea of virtual work may seem strange at first. But it's one of the most fascinating I've encountered.
The problem I had at first with the idea of a set of virtual forces was the "definition" given in most textbooks. Usually, it goes something like this: "In this statement, the term virtual means the forces are not related to the deformations of the body."
But, as far as I know, the forces applied to a body are always related to the deformations. This relationship may be more or less complicated, but it's a relationship that exists. This made me think that virtual forces are forces that are virtual or imaginary in a litteral sense. We imagine them to be there, but they're actually not there. So, whenever you find a "definition" such as the one you find above, all you'll be reading is something like: "a virtual force is a force that is virtual".
It's a childish idea (in the best sense of the word childish), but many applications show it to be a very powerfull one: amazingly, it works.
Would you agree with meaning of virtual force such as I wrote it?