...not only a structure of great size by...also a structure which is frequently:
1 :: constructed of modular units;
2 :: capable of great or even 'unlimited' extension;
3 :: a structural framework into which smaller structural units (for example, rooms, houses, or small buildings of other sorts) can be built – or even plugged-in or clipped-on, having been prefabricated elsewhere;
4 :: a structural framework expected to have a useful life much longer than that of the smaller units which it might support.
Banham's introduction also features other definitions:
Kenzo Tange (1964)
"Mega-structure is a large frame in which all the functions of a city or part of a city are housed. It has been made possible by present day technology. In a sense it is a man-made feature of a landscape. It is like the great hill on which Italian towns were built."
"Urban Structures for the future"