Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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