Someone who pretends to be someone else, especially on the internet. Found on anywhere from instagram to twitter to chat sites, these people use fake pictures to disguise who they are. A synonym for this in some situations may be "troll" because the majority of catfish out there are simply out to troll others, while others have their own reasons for this approach. Often catfish, once discovered, are faced with people full of annoyance and frustration at being "catfishes".
Toward the end of Catfish the documentary, the movie’s protagonist Nev Schulman coins the term after talking to his personal catfish’s husband, who used to be a fisherman.
“They used to tank cod from Alaska all the way to China. They’d keep them in vats in the ship. By the time the codfish reached China, the flesh was mush and tasteless. So this guy came up with the idea that if you put these cods in these big vats, put some catfish in with them and the catfish will keep the cod agile. And there are those people who are catfish in life. And they keep you on your toes.”