A megastructure is a very large artificially created object, though the limits of precisely how large this is vary considerably. Some apply the term to any especially large or tall building.
Some sources define a megastructure as an enormous self-supporting artificial construct. Other criteria such as rigidity are sometimes also applied, so large clusters of associated smaller structures may or may not qualify. The products of megascale engineering or astroengineering are megastructures.