MegaCity is the unofficial name for the latest Amalgamation of City of Toronto, comprised of the former cities of Old Toronto, North York, Etobicoke, City of York, Scarborough, and the borough of East York plus regional municipality of Toronto, the Metro level of government.
It was used and heard often in the years leading up to and immediately after Amalgamation in 1998.
Using MegaCity when refering to the entire (new) City of Toronto helped alleviate some of the confusion when you were talking about the Old City of Toronto in the late 1990s and early 2000s.
Nowadays, Megacity and Toronto are interchangeable as most people have accepted, if not embraced, the concept of an Amalgamated City.