The Collaborative International Dictionary of English v.0.48:

Laudanum \Lau"da*num\, n. [Orig. the same wort as ladanum,
   ladbdanum: cf. F. laudanum, It. laudano, ladano. See
   Ladanum.]
   Tincture of opium, used for various medical purposes.
   [1913 Webster]

   Note: A fluid ounce of American laudanum should contain the
         soluble matter of one tenth of an ounce avoirdupois of
         powdered opium with equal parts of alcohol and water.
         English laudanum should have ten grains less of opium
         in the fluid ounce. --U. S. Disp.
         [1913 Webster]

   Dutchman's laudanum (Bot.) See under Dutchman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

laudanum
    n 1: narcotic consisting of an alcohol solution of opium or any
         preparation in which opium is the main ingredient [syn:
         laudanum, tincture of opium]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "laudanum":
   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,
   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, liquor, lotus, meperidine,
   methadone, morphia, morphine, narcotic, opiate, opium, pacifier,
   pain killer, paregoric, pen yan, phenobarbital,
   phenobarbital sodium, purple heart, quietener, rainbow, red, scag,
   secobarbital sodium, sedative, shit, sleep-inducer, sleeper,
   sleeping draught, sleeping pill, smack, sodium thiopental,
   somnifacient, soother, soothing syrup, soporific, tar,
   tranquilizer, turps, white stuff, yellow, yellow jacket