A megagame is a growing genre of games that have the following features:
Teams of players, sometimes organised into a hierarchy of teams
Megagames have been compared to LARPs and there is much similarly, particularly in the context of role-playing. However in megagames the emphasis is on a structured game environment and ensuing that each player is a key part of the narrative. There is also no requirement to dress up (though many megagamers add to their roles in this way on occasion).
Megagames have a very wide range of subject matter - for many years the UK group, Megagame Makers has run games on a big selection including operational military subjects, large POL/MIL games, diplomatic and treaty-making games, UN International crisis games, SF interstellar politics and warfare, historical crises of various types.
To find out more about the range of subjects being covered by Megagame Makers in the UK, visit their site here
Now, since the emergence of megagames into the wider world, there are a whole host of organisations putting on megagames globally, for more details click here.