The Collaborative International Dictionary of English v.0.48:

chloral hydrate \chloral hydrate\ n.
   1. a chemical substance (CCl3.CH(OH)2) which is a hydrate
      of trichloroacetaldehyde. It crystallizes as white
      monoclinic plates, obtained by treating chloral with
      water. It produces sleep when taken internally or
      hypodermically, and is used in medicine as a hypnotic and
      sedative; -- called also chloral

   Note: It may be habit-forming, and is a controlled substance
         listed in the U. S. Code of Federal Regulations. It is
         sometimes used to render a person unconscious for
         illegal or nefarious purposes, and in this use, a
         concentrated solution is one of the agents called
         knockout drops.
         [PJC]

WordNet (r) 3.0 (2006):

chloral hydrate
    n 1: a colorless crystalline drug used as a sedative; irritates
         the stomach and can be addictive