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

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "basement":
   WC, archives, armory, arsenal, attic, backhouse, bank, base,
   baseboard, basis, bathroom, bay, bearing wall, bed, bedding,
   bedrock, bin, bonded warehouse, bookcase, bottom, box, bunker,
   buttery, can, cargo dock, cellar, cellarage, chassis, chest,
   closet, coal bin, comfort station, conservatory, convenience,
   crapper, crate, crib, cupboard, cyclone cellar, dado, depository,
   depot, dock, drawer, dump, earth closet, exchequer, floor,
   flooring, fond, foot, footing, foundation, frame, fundament,
   fundamental, glory hole, godown, ground, grounds, groundwork,
   hardpan, head, hold, hole, hutch, john, johnny, johnny house, keel,
   latrine, lavatory, library, locker, lumber room, lumberyard,
   magasin, magazine, mopboard, nadir, necessary, outhouse, pavement,
   potato cellar, powder room, principle, privy, rack, radical,
   repertory, repository, reservoir, rest room, rick, riprap,
   rock bottom, rudiment, seat, shelf, shoemold, sill, sole,
   solid ground, solid rock, stack, stack room, stereobate,
   stock room, storage, store, storehouse, storeroom, storm cellar,
   stylobate, subbasement, substratum, substruction, substructure,
   supply base, supply depot, tank, terra firma, toe, toilet,
   toilet room, treasure house, treasure room, treasury,
   underbuilding, undercarriage, undergirding, underpinning,
   understruction, understructure, urinal, vat, vault, wainscot,
   warehouse, washroom, water closet, wine cellar