The Collaborative International Dictionary of English v.0.48:

Paraldehyde \Par*al"de*hyde\, n. [Pref. para- + aldehyde.]
   (Chem.)
   A trimer of acetaldehyde (C6H12O3), prepared by
   polymerization of acetaldehyde with hydrochloric and sulfuric
   acids. It has sedative and hypnotic properties. --MI11
   [PJC]

WordNet (r) 3.0 (2006):

paraldehyde
    n 1: a colorless liquid (a cyclic trimer of acetaldehyde) that
         is used as a sedative and a solvent [syn: paraldehyde,
         ethanal trimer]