RhymeZone: megastructure definitions

Definitions of megastructure:

noun: (architecture) A specific type of structure characterized by very large buildings with strictly artificial forms.
noun: (science fiction) A hypothetical structure not possible with current technology, e.g., a Dyson sphere.
noun: (uncountable) The large-scale structure of a material, as contrasted to the fine-scale structure.

(Definitions from Wiktionary)