megastructure

Short definition from Wiktionary

Noun

1. 1
   A very large structure.
2. 2
   A very large structure: A specific type of structure characterized by very large buildings with strictly artificial forms.
3. 3
   A very large structure: A hypothetical structure not possible with current technology, e.g., a Dyson sphere.
4. 4
   The large-scale structure of a material, as contrasted to the fine-scale structure.