From Longman Dictionary of Contemporary Englishbreastbonebreast‧bone /ˈbrestbəʊn $ -boʊn/ noun [countable] 
HBHa long flat bone in the front of your chest that is connected to the top seven pairs of ribs SYN sternum
HBHa long flat bone in the front of your chest that is connected to the top seven pairs of ribs SYN sternumExamples from the Corpus
breastbone• It has tender meat; soft, pliable, smooth-textured skin; and flexible breastbone cartilage. 3.• It has tender flesh; smooth-textured, soft, and pliable skin; and a somewhat flexible breastbone cartilage. 3.• The doctor found that her breastbone was loose.• The ache in her head got worse and she began to imagine a pain under her breastbone.• His rib cage was missing on one side where a large piece of shell casing stuck out from under his breastbone.• To put the tissue behind the breastbone, where it grows naturally, would have meant breaking bone.• Terror suddenly hit her like an iron arrow in the breastbone and her throat seized up and she choked.• Open the back, and with the heel of your hand, press the center of the breastbone firmly to flatten it. 