Your sternum is your breastbone, the flat plate at the top of your rib cage. Mammals and birds — and even arachnids, or spiders — have a sternum.
If you study human anatomy, you'll learn that the sternum is a long, flat, T-shaped bone made of fused segments. The rib cage attaches the sternum to the spine, or vertebrae. The word itself comes from the Greek sternon, "chest, breast, or breastbone," from a root that means "flat surface."