WordNet (r) 3.0 (2006):

tranquillise
    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]