What is the difference between virtual object and virtual image?
The idea which helped me to differentiate between them is as follows: Image is a point of intersection of reflected/refracted rays. They might actually intersect (real image) or one might have to geometrically produce them to get them to intersect (virtual image). One might always find that the real image is formed if this point of intersection is on the reflected/ refracted side.
An object is a point of intersection of incident rays. Again, if this point of intersection lies on the incident side, it will be real; but if it lies on the other side , it will be virtual. Treating an object as virtual helps us to get allocate an object when physically there is none, as in the case of light rays converging towards a lens or mirror.