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 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 ]