breastbone(n = noun.body) sternum - the flat bone that articulates with the clavicles and the first seven pairs of ribs;
is a kind of bone, os
is a part of chest, pectus, thorax, axial skeleton
has parts: corpus sternum, gladiolus, manubrium, xiphoid process

   The bone of the breast; the sternum. [1913 Webster]

breastbone, n. a thin flat vertical bone and cartilage in the chest connecting the ribs.