Don't get stuck at the word "virtual", it has nothing to do with the image not being a well defined thing in the physical world. It is just (arbitrary) nomenclature.
A real image is one that can be projected onto a screen (that is, where the light coming from one point of the object is approximately collected in one point in space).
In a virtual image the light propagates as if there was an object or real image at the position of the virtual image, but if you put a screen at the position of the virtual image you cannot see it there (because, for example, in the case of virtual image due to a concave lens the virtual image is before the lens and the light ray have not been deflected yet).
This also tells you that you can form a real image from a virtual image, since the light rays behave as if there was an object at the position of the virtual image, so we can place a lens which forms a real image of the virtual image.
Actually, exactly that happens when a human sees a virtual image (which happens, for example, when using a magnifying glass), the lens in the eye projects a real image of the virtual image onto the retina.