Sternum: The long flat bone in the upper middle of the front of the chest. The sternum articulates (comes together) with the cartilages of the first seven ribs and with the clavicle (collarbone) on either side. The sternum consists of three parts: the manubrium (the upper segment of the sternum, a flattened, roughly triangular bone), the corpus (body) of the sternum, and the xiphoid process (the little tail of the sternum than points down). These sections of the sternum arise as separate bones, and they may fuse partially or completely with one another. Also known as breastbone.