sternum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A long flat bone in most vertebrates that is situated along the ventral midline of the thorax and articulates with the ribs. The manubrium of the sternum articulates with the clavicles in humans and certain other vertebrates. Also called breastbone.

Century Dictionary and Cyclopedia

1. n. The breast-bone of man and many other vertebrates; a bone or longitudinal series of bones in the middle line of the ventral aspect of the body, chiefly in its thoracic section, completing the thoracic wall by articulation with more or fewer ribs, or elements of the scapular arch, or both: theoretically, in Owen's system, the hemal spines of a series Of vertebræ. In man and most mammals the sternum consists of an anterior piece, the “handle,” manubrium, or presternum; of several (in man four) segments or sternebers constituting the body of the sternum, gladiolus, or mesosternum; and of a terminal piece, the xiphoid or ensiform cartilage, or xiphisternum. It articulates in man with the clavicles and with seven costal cartilages. The sternebers of a mammalian sternum may remain perfectly distinct, or be ankylosed in one. (See cut under mesosternum.) In cetaceans and sirenians the sternum is much reduced, and may be a single bone or quite rudimentary. In the monotrematous mammals a small median bone called proösteon is developed in front of the præsternum. The parts called episternum, omosternum, interclavicle, in the mammals just mentioned, or in various reptiles, or in batrachians, belong rather to the shoulder-girdle. There is no sternum in some reptiles, as serpents. See cuts under Catarrhina, Elephantinæ, interclavicle, omosternum, and skeleton.
2. n. In arthropods, as insects and crustaceans, a median sternal or ventral sclerite of any somite of the cephalothorax, thorax, or abdomen; a sternite: the opposite of a tergite or notum. In such cases, sternum and sternite are used interchangeably, sternum being seldom used of the series of sternites as a whole. (See cut under cephalothorax.) In insects the three thoracic sterna are specified as prosternum, mesosternum, and metasternum. In Diptera, sternum, generally means the mesosternum, as the other thoracic rings do not show a sternal piece. In Coleoptera, sternum is sometimes extended to include the episterna and epimera, or whole lower surface of a thoracic segment. See episternum, 3.

Wiktionary

1. n. The breastbone.

GNU Webster's 1913

1. n. (Anat.) A plate of cartilage, or a series of bony or cartilaginous plates or segments, in the median line of the pectoral skeleton of most vertebrates above fishes; the breastbone.
2. n. (Zoöl.) The ventral part of any one of the somites of an arthropod.

WordNet 3.0

1. n. the flat bone that articulates with the clavicles and the first seven pairs of ribs