The Collaborative International Dictionary of English v.0.48:

counterclockwise \coun"ter*clock"wise\ adj. & adv.
   in the opposite direction to that in which the hands of a
   clock rotate, as viewed from in front of the clock face; --
   of rotatory motion or spiral direction. Opposite of
   clockwise, or right-handed

   Note: [Narrower terms: sinistral levororotary,
         levorotatory]

   Syn: left-handed.
        [PJC]

WordNet (r) 3.0 (2006):

counterclockwise
    adv 1: in a direction opposite to the direction in which the
           hands of a clock move; "please move counterclockwise in a
           circle!" [syn: counterclockwise, anticlockwise] [ant:
           clockwise]
    adj 1: in the direction opposite to the rotation of the hands of
           a clock [syn: counterclockwise, anticlockwise,
           contraclockwise] [ant: clockwise]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "counterclockwise":
   a rebours, a reculons, against the grain, anticlockwise, arear,
   around, ass-backwards, astern, away, back, backward, backwards,
   clockwise, earthward, fro, head over heels, heavenward,
   heels over head, hindward, hindwards, homeward, in a circle,
   in a spin, in a whirl, in circles, in reverse, landward, larboard,
   leeward, left, left-hand, left-wing, left-wingish, leftward,
   levorotatory, liberal, near, nigh, port, radical, rearward,
   rearwards, retrad, rightward, round, round about, round and round,
   seaward, sinister, sinistral, sinistrocerebral, sinistrocular,
   sinistrogyrate, sinistrorse, widdershins, windward