The sternum is the flat bone down the middle of your chest, at the front. Put your hand between your two collarbones and slide it downwards — that hard, flat strip is it. Its job is to hold the front ends of your ribs together, so your ribs form a cage around your heart and lungs instead of hanging loose. In plain conversation, English speakers say breastbone; sternum is the medical word, what a doctor, nurse or anatomy book uses. Both are perfectly correct — the choice tells you the setting, not the meaning. You will meet sternum most often in three places: first aid classes (CPR pushes on the lower half of it), sports and gym talk (chest exercises, 'bring the bar to your sternum'), and hospital talk about heart surgery, where the surgeon must cut through it to reach the heart. The plural is sternums in ordinary use, or sterna if you want the Latin form, which biologists prefer. There is also an adjective, sternal, as in 'sternal pain'. Small anatomy bonus: the little pointed tip at the bottom of it has its own name, the xiphoid process, and first-aid teachers warn you not to press on it.