The Collaborative International Dictionary of English v.0.48:

bathtub \bath"tub\ n.
   A relatively large tub used to take a bath, usually a
   permanent fixture in a bathroom; it is an open container that
   is filled with water, in which a person immerses himself for
   the purpose of washing the body.

   Syn: bath, tub.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

bathtub
    n 1: a relatively large open container that you fill with water
         and use to wash the body [syn: bathtub, bathing tub,
         bath, tub]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "bathtub":
   aquamanile, automatic dishwasher, basin, bath, bidet, dishpan,
   dishwasher, ewer, finger bowl, kitchen sink, lavabo, lavatory,
   piscina, shower, shower bath, shower curtain, shower head,
   shower room, shower stall, showers, sink, tub, wash barrel,
   wash boiler, washbasin, washbowl, washdish, washer,
   washing machine, washing pot, washpot, washstand, washtub