WordNet (r) 3.0 (2006):

quieten
    v 1: become quiet or quieter; "The audience fell silent when the
         speaker entered" [syn: quieten, hush, quiet,
         quiesce, quiet down, pipe down] [ant: louden]
    2: 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]
    3: cause to be quiet or not talk; "Please silence the children
       in the church!" [syn: hush, quieten, silence, still,
       shut up, hush up] [ant: louden]