Free Technology 4 Teachers. This is a site that defines words via videos. While it is not really a dictionary in video format it, it is more like videos that contain that word being searched. These videos can be commented and rated and come from YouTube. I would only use this as a teacher by embedded/saving the video and showing it that way, not by having students go to it themselves (typical YouTube safety rules, unless you use YouTube for Schools).
Check out Vifinition by clicking here!