The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pronation
    n 1: rotation of the hands and forearms so that the palms face
         downward [ant: supination]