What is a Megadungeon

 What is a megadungeon? 

There is various definitions that are passed around -- I might go into more detail later on the debate, but I will simply state what I am using as a definition.

A megadungeon is a dungeon that one can operate a long running campaign with it as the basis of the campaign.   It is the keystone of the campaign, and by it very nature and the fact that it not static means that it never truly be cleared.  

(though I expect to offer some overland fun to provide breaks when the players don't want to delve that night.)