The Collaborative International Dictionary of English v.0.48:

Apartment \A*part"ment\, n. [F. appartement; cf. It.
   appartamento, fr. appartare to separate, set apart; all fr.
   L. ad + pars, partis, part. See Apart.]
   1. A room in a building; a division in a house, separated
      from others by partitions. --Fielding.
      [1913 Webster]

   2. A set or suite of rooms. --De Quincey.
      [1913 Webster]

   3. A compartment. [Obs.] --Pope.
      [1913 Webster]

   4. A room or suite of rooms in a building comprising a
      dwelling unit separate from others in the building, and
      typically having its own separate bath, sanitary, and
      kitchen facilities. Such apartments are in most cases
      rented from the owner by those dwelling in them.
      [PJC]

   efficiency apartment, a small apartment[4], sometimes
      furnished, with minimal kitchen and bath facilities. The
      unit may comprise a single room plus a bathroom, and the
      kitchen facilities are often open to the main room, or may
      form a small niche in a corner. There are many variations
      of efficiency apartment, including some in which
      furnishings such as a bed may be pull out from a wall
      recess and stored there again when not in use. Also called
      an efficiency.
      [PJC]

WordNet (r) 3.0 (2006):

apartment
    n 1: a suite of rooms usually on one floor of an apartment house
         [syn: apartment, flat]