WordNet (r) 3.0 (2006):

deuterium
    n 1: an isotope of hydrogen which has one neutron (as opposed to
         zero neutrons in hydrogen) [syn: deuterium, heavy
         hydrogen]

The Elements (07Nov00):

deuterium
Symbol: D
Atomic Weight: 2
A hydrogen atom which has but one proton and neutron. Deuterium
makes up
about 0.015% of all natural hydrogen. Chemical properties are like that
of
normal hydrogen, though somewhat slower.