The Collaborative International Dictionary of English v.0.48:

Basement \Base"ment\ (b[=a]sment), 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.
       [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

basement
    n 1: the lowermost portion of a structure partly or wholly below
         ground level; often used for storage [syn: basement,
         cellar]
    2: the ground floor facade or interior in Renaissance
       architecture