noun

• A very large structure.
  "A fusion of architecture and city planning, megastructures were supposed to bring together mixed uses -- like housing, shopping, transportation and public space -- within a single building."
• A very large structure.
  "For lack of anything intelligent to do (it's a slow weekend), I've been compiling a list of stories about "megastructures" -- ringworlds, Dyson spheres, and so forth."
• (uncountable) The large-scale structure of a material, as contrasted to the fine-scale structure.
  "That local concentration of the sulphur, phosphorous and carbon is also found in many other positions has been proved by the most invaluable method of examining the macrostructure or megastructure of complete sections, of ingots and forged material, after the surfaces of the steel have been polished and etched in a suitable way."