A name taken from, primarily, US Worldcons, for the large-scale key items of a convention.
These usually include the Opening and Closing ceremonies, the Masquerade/Fancy Dress competition, and any large scale entertainments such as plays, orchestras and other items that may require co-ordination across multiple departments and usually access to the main hall to set up/tear down. The disco/ceilidh is not usually considered to be an extravaganza but something like a Tesla coil demonstration or a Sound+Light (Explosions) item might be.
At LX2009 it was suggested that a named person be made responsible for the cross-team co-ordination and communication required for these large items. This person may also be the Stage Manager or Producer of said items.