Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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