What does "basement" mean?

Basement: The outer wall of the ground story of a building, or of a part of that story, when treated as a distinct substructure. ( See Base, n., 3 (a).) Hence: The rooms of a ground floor, collectively. Basement membrane (Anat.), a delicate membrane composed of a single layer of flat cells, forming the substratum upon which, in many organs, the epithelioid cells are disposed.