Source : Webster's Revised Unabridged Dictionary (1913)

Tranquilizer \Tran"quil*i`zer\, Tranquillizer \Tran"quil*li`zer\
,  n.
   One who, or that which, tranquilizes.

Source : WordNet®

tranquillizer
     n : a drug used to reduce stress or tension without reducing
         mental clarity [syn: {tranquilizer}, {tranquilliser}, {antianxiety
         agent}, {ataractic drug}, {ataractic agent}, {ataractic}]