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]