A plot of an episode or story arc wherein one or more villainous characters -- usually made up of some combination of past one-shots, members of the Rogues Gallery, and/or a few possible newcomers -- join forces to destroy their common enemies, the heroes.
Usually, they will initially overpower the heroes, but their own evil selfishness and inability to trust each other undermines their unified front, and the heroes strike at this weakness. Or one dominant personality double-crosses the others. Either way, the team dissolves at the end.
This is often saved for a season or series finale. Crossovers do this a lot, using one villain from each series.
Named after the nemesis organization to the Superfriends cartoon of the 1970s, who actually managed to stay together, mostly due to relentless use of a Deus Ex Machina Reset Button to escape in the last scene.