The Collaborative International Dictionary of English v.0.48:
heroin \her"o*in\ (h[~e]r"[-o]*[i^]n), n. (Chem.)
a morphine derivative, diacetyl morphine, used to relieve
severe pain and as a sedative. It is highly addictive, and
its use is strictly controlled in the U.S. by federal law. It
is a popular strong narcotic drug of abuse, in part because
it is more soluble than morphine. It is sometimes included as
one of the components of Brompton's mixture, used to control
pain in terminallly ill patients.
Syn: diacetyl morphine, H, horse, junk, scag, shit, smack.
[WordNet 1.5 +PJC]