The Collaborative International Dictionary of English v.0.48:

Breastbone \Breast"bone`\ (br[e^]st"b[=o]n`), n.
   The bone of the breast; the sternum.
   [1913 Webster]

WordNet (r) 3.0 (2006):

breastbone
    n 1: the flat bone that articulates with the clavicles and the
         first seven pairs of ribs [syn: sternum, breastbone]