Source : Webster's Revised Unabridged Dictionary (1913)

Pronation \Pro*na"tion\, n. [Cf. F. pronation.] (Physiol.)
   (a) The act of turning the palm or palmar surface of the
       forefoot downward.
   (b) That motion of the forearm whereby the palm or palmar,
       surface is turned downward.
   (c) The position of the limb resulting from the act of
       pronation. Opposed to {supination}.

Source : WordNet®

pronation
     n : rotation of the hands and forearms so that the palms face
         downward [ant: {supination}]