The Collaborative International Dictionary of English v.0.48:

Xiphisternum \Xiph"i*ster"num\, n.; pl. Xiphisterna. [NL., fr.
   Gr. xi`fos a sword + sternum.] (Anat.)
   (a) The posterior segment, or extremity, of the sternum; --
       sometimes called metasternum, ensiform cartilage,
       ensiform process, or xiphoid process.
   (b) The xiphiplastron. -- Xiph"i*ster"nala.
       [1913 Webster]

WordNet (r) 3.0 (2006):

xiphoid process
    n 1: smallest of the three parts of the breastbone; articulates
         with the corpus sternum and the seventh rib