Here's my short answer, assuming that "levels" and "experience points" are relevant in your game.
- It is a single adventuring site with multiple areas of increasing difficulty (challenge levels)
- with enough "experience points" (rewards of adventuring relevant to character advancement) in each "challenge level"
- that two or more adventuring parties can advance to the character level needed to confront the next challenge level, without intruding on each other's sources of experience.
In other words, if in your game a single party will level up after exploring 30 rooms, then each level should have 60 rooms or more to be a megadungeon. To be clear, my definition is not so much about whether the megadungeon literally takes on multiple parties, but more about whether a single party feels that they have a great deal of freedom to get to the next level in multiple ways.
|Anonymous, from plagmada,org
Compare this to the more typical adventure-based campaign where each individual adventure site gives all or part of the experience to advance one level. In that kind of campaign, multiple parties can coexist by visiting different adventure sites, instead of the same one.
Right now I'm running one of each kind of campaign and they each have their own rewards -- the multi-site campaign has a lot of breadth and variety while the single-site campaign offers intensity and the development of a strange, obsessive legendry over multiple visits.