Bump/signed. As for definition: you have to have the item to use the appearance. Example, i have a blue helm and Saint Helm 14 in my inventory. With transmog i can equip the blue helm but make a selection to make it appear like it is Saint Helm 14. All you change is appearance. I think most people understand what it is, but yeah the two main things are 1) only changing appearance, keeping stats and, 2) you must have the item in order to transmog.