MegaCrossover

A crossover is a piece of FanFic which involves characters, plot elements, or what-have-you from two or more different works, such as BubblegumCrisis? and RanmaOneHalf?, or StarTrek and BabylonFive?.

A MegaCrossover is like that, only moreso, crossing multiple works (from four or five on up) and including many of the works central to the genre (in the case of anime, anyhow). Also, whereas a crossover may focus mainly on one of the works being crossed (say, one or two characters from one series get dropped into another, where they help seriously mess up the plot of the original), a MegaCrossover doesn't focus as much on any particular series. Examples of anime MegaCrossovers include UndocumentedFeatures, TogetherAgain?, and DanceOfShiva? (there are undoubtedly more; I just can't think of any of them).