From WordNet (r) 3.0 (2006) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]