Wordnet 3.0
NOUN (1)
1.
a bed with a mattress made of strong plastic that is filled with water;
The Collaborative International Dictionary of English v.0.48:
Water bed \Wa"ter bed`\
A kind of mattress made of, or covered with, waterproof
fabric and filled with water, thus providing uniform support
to the body lying on it; -- also a bed constructed so as to
contain such a mattress. It is used in hospitals for
bedridden patients, as well as in some private homes.
[1913 Webster +PJC]
WordNet (r) 3.0 (2006):
water bed
n 1: a bed with a mattress made of strong plastic that is filled
with water