The Collaborative International Dictionary of English v.0.48:

outhouse \out"house`\, n.
   1. A small house or building at a little distance from the
      main house; an outbuilding.
      [1913 Webster]

   2. Especially: A small building with one or more seats and a
      pit underneath, intended for use as a toilet; a privy.
      [PJC]

WordNet (r) 3.0 (2006):

outhouse
    n 1: a small outbuilding with a bench having holes through which
         a user can defecate [syn: outhouse, privy, earth-
         closet, jakes]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "outhouse":
   Nissen hut, Quonset hut, WC, backhouse, basement, bathroom, booth,
   can, closet, comfort station, convenience, crapper, crib,
   earth closet, gatehouse, head, hut, hutch, jakes, john, johnny,
   johnny house, kiosk, latrine, lavatory, lean-to, necessary, office,
   outbuilding, pavilion, powder room, privy, rest room, sentry box,
   shack, shanty, shed, stall, toilet, toilet room, tollbooth,
   tollhouse, urinal, washroom, water closet