The Collaborative International Dictionary of English v.0.48:

Double-decker \Dou"ble-deck"er\, n.
   1. (Naut.)

   1. A man-of-war having two gun decks.
      [1913 Webster]

   2.
      (a) A tenement house having two families on each floor.
          [Local, U. S.]
      (b) A biplane a["e]roplane or kite. [Colloq.]
          [Webster 1913 Suppl.]

   3. anything having two decks or levels, as a bus with two
      levels for passengers. In older vehicles, less often
      today, the upper deck was open, as on street cars.
      [Colloq.]
      [PJC]

WordNet (r) 3.0 (2006):

double-decker
    n 1: a vehicle carrying many passengers; used for public
         transport; "he always rode the bus to work" [syn: bus,
         autobus, coach, charabanc, double-decker, jitney,
         motorbus, motorcoach, omnibus, passenger vehicle]