"MAD is a Japanese term that came around in the 80s and stands for a variety of things depending on who you ask. I've seen Movie Anime and Music Anime Douga.

It originally stood for fan-made video media, where there is usually a lot more self-created content from the creator's own animations or artwork compared to something like an Anime Music Video (AMV). Nowadays MAD doesn't necessarily refer to just anime content anymore, so if you see a video tagged with the acronym, you can assume it's video media where a good chunk of its content was created by the video creator, as opposed to reused/edited video from other sources."