Megastructures are artificial constructs so huge that they challenge our preconceptions of size.
A megastructure, by definition, has to be a single, unified structure. A modern city may be huge in scale, but its is actually composed of many thousands of individual buildings, all built at different times for different purposes. The Great Wall of China may be considered a megastructure, however, as it is a single complete structure with one dedicated purpose.
Megastructures may be intelligent life's ultimate expression of existence, as they challenge the very scale of the universe.
Stellar Scale Megastructures are constructs built on the scale of stars.
Stellar-scale O'Neill Colonies.
Cosmic rings spinning in the void.
Englobing entire stars.