The Collaborative International Dictionary of English v.0.48:

Opium \O"pi*um\, n. [L., fr. Gr. ? poppy juice, dim. of ?
   vegetable juice.] (Chem.)
   The inspissated juice of the Papaver somniferum, or white
   poppy.
   [1913 Webster]

   Note: Opium is obtained from incisions made in the capsules
         of the plant, and the best flows from the first
         incision. It is imported into Europe and America
         chiefly from the Levant, and large quantities are sent
         to China from India, Persia, and other countries. It is
         of a brownish yellow color, has a faint smell, and
         bitter and acrid taste. It is a stimulant narcotic
         poison, which may produce hallicinations, profound
         sleep, or death. It is much used in medicine to soothe
         pain and inflammation, and is smoked as an intoxicant
         with baneful effects.
         [1913 Webster]

   Opium joint, a low resort of opium smokers. [Slang]
      [1913 Webster]

WordNet (r) 3.0 (2006):

opium
    n 1: an addictive narcotic extracted from seed capsules of the
         opium poppy

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "opium":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Seconal, Seconal pill,
   Tuinal, Tuinal pill, alcohol, amobarbital sodium, analgesic,
   anesthetic, anodyne, barb, barbiturate, barbiturate pill,
   black stuff, blue, blue angel, blue devil, blue heaven,
   blue velvet, calmative, chloral hydrate, codeine,
   codeine cough syrup, depressant, depressor, dolly, downer,
   goofball, hard stuff, heroin, hop, horse, hypnotic, junk,
   knockout drops, laudanum, liquor, lotus, lullaby, mandrake,
   meperidine, methadone, morphia, morphine, narcotic, nightcap,
   opiate, pacifier, pain killer, paregoric, pen yan, phenobarbital,
   phenobarbital sodium, poppy, purple heart, quietener, rainbow, red,
   scag, secobarbital sodium, sedative, shit, sleep-bringer,
   sleep-inducer, sleep-producer, sleeper, sleeping draught,
   sleeping pill, smack, sodium thiopental, somnifacient, soother,
   soothing syrup, soporific, tar, tranquilizer, turps, white stuff,
   yellow, yellow jacket