1. The most disturbing phenomenon of our generation.
2. A group of people all obsessed with the same thing. Usually a work of fiction being either a movie, book, TV show, anime, comic, video game, etc...
3. A way for lonely people to feel a part of something larger than themselves.
4. A collective of creepy obsessive type people that only serves to feed and encourages each other in unhealthy obsessions and exaggerated exultation of non-existent people, creatures, places and objects.
"Oh my god, I wanted to search for pictures of that actor from that movie we saw last night but I was inundated with terrible text posts and reaction gifs from a fandom I didn't even know existed! Ugh!"