The Collaborative International Dictionary of English v.0.48:

Sternum \Ster"num\, n.; pl. L. Sterna, E. Sternums. [NL.,
   from Gr. ?, the breast, chest.]
   1. (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.
      [1913 Webster]

   Note: The sternum is connected with the ribs or the pectorial
         girdle, or with both. In man it is a flat bone, broad
         anteriorly, narrowed behind, and connected with the
         clavicles and the cartilages of the seven anterior
         pairs of ribs. In most birds it has a high median keel
         for the attachment of the muscles of the wings.
         [1913 Webster]

   2. (Zool.) The ventral part of any one of the somites of an
      arthropod.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sternum
    n 1: the flat bone that articulates with the clavicles and the
         first seven pairs of ribs [syn: sternum, breastbone]