Source : Webster's Revised Unabridged Dictionary (1913)

Tranquilize \Tran"quil*ize\, Tranquillize \Tran"quil*lize\, v.
   t. [imp. & p. p. {Tranquilized}or {Tranquilliized}; p. pr. &
   vb. n. {Tranquilizing}or {Tranquillizing}.] [Cf. F.
   tranquilliser.]
   To render tranquil; to allay when agitated; to compose; to
   make calm and peaceful; as, to tranquilize a state disturbed
   by factions or civil commotions; to tranquilize the mind.

   Syn: To quiet; compose; still; soothe; appease; calm; pacify.

Source : WordNet®

tranquilize
     v 1: make calm or still; "quiet the dragons of worry and fear"
          [syn: {calm}, {calm down}, {quiet}, {tranquillize}, {tranquillise},
           {quieten}, {lull}, {still}] [ant: {agitate}]
     2: cause to be calm or quiet as by administering a sedative to;
        "The patient must be sedated before the operation" [syn: {sedate},
         {calm}, {tranquillize}, {tranquillise}] [ant: {stimulate}]