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.
Cosmic Scale Megastructures are constructs built on a scale greater than stars. They can approach the size of solar systems, galaxies, and beyond.
A custom-made cosmos.
A loop of cosmic string ten million light years across.