Basement \Base"ment\, n. [F. soubassement. Of uncertain origin. Cf. Base, a., Bastion.]

(Arch.) 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.

Source: Webster's Revised Unabridged Dictionary (1913)