The Collaborative International Dictionary of English v.0.48:

tetrahydrocannabinol \tet`ra*hy`dro*can*nab"i*nol\
   (t[e^]t`r[.a]*h[imac]`dr[-o]*k[a^]n*n[a^]b"[i^]n[add]l), n.
   (Chem. & Med.)
   A chemical substance (C21H20O2) which is produced by the
   hemp plant (Cannabis sativa), and is the physiologically
   active agent of dried preparations of that plant, called
   variously marijuana, hashish, ganja, hemp, etc.; also called
   THC. It causes the euphoric effect for which the
   preparations are smoked or chewed. It is used in medicine in
   a purified form as an antiemetic (an antinausea agent),
   especially in conjunction with chemotherapy of cancer. It
   occurs primarily as the [Delta]1-3,4-trans isomer, also
   called [Delta]9-THC, with small amounts of the
   [Delta]6-3,4-trans isomer detectable at about 1%. It is a
   controlled substance, classified as a hallucinogen, and its
   possession or distribution is illegal in almost all states of
   the United States. --MI11
   [PJC]