The Collaborative International Dictionary of English v.0.48:

Depressant \De*press"ant\, n. (Med.)
   An agent or remedy which lowers the vital powers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

depressant
    adj 1: capable of depressing physiological or psychological
           activity or response by a chemical agent [ant:
           stimulative]
    n 1: a drug that reduces excitability and calms a person [syn:
         sedative, sedative drug, depressant, downer]