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]