The Collaborative International Dictionary of English v.0.48:

Tranquilize \Tran"quil*ize\, Tranquillize \Tran"quil*lize\, v.
   t. [imp. & p. p. Tranquilizedor Tranquilliized; p. pr. &
   vb. n. Tranquilizingor 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.
   [1913 Webster]

   Syn: To quiet; compose; still; soothe; appease; calm; pacify.
        [1913 Webster] Tranquilizer

WordNet (r) 3.0 (2006):

tranquillize
    v 1: make calm or still; "quiet the dragons of worry and fear"
         [syn: calm, calm down, quiet, tranquilize,
         tranquillize, tranquillise, quieten, lull, still]
         [ant: agitate, charge, charge up, commove,
         excite, rouse, turn on]
    2: cause to be calm or quiet as by administering a sedative to;
       "The patient must be sedated before the operation" [syn:
       sedate, calm, tranquilize, tranquillize,
       tranquillise] [ant: arouse, brace, energise,
       energize, perk up, stimulate]