Question

We define an image formed by a plane mirror to be a 'Virtual' image.

Answer 1

Lets re-iterate what we told you in the hint once again. The image of an object is, where the light rays coming from the object after going through a change meet or intersect. Remember in the case of a plane mirror the change that the rays coming from the object will go through is "reflection".
Does a virtual image mean : Virtual intersection of those reflected rays ?
Yes!
So when the reflected rays do not meet in real, but appear to be meeting at any point (A virtual interaction indeed!!) we define my friends, a virtual image.