The Collaborative International Dictionary of English v.0.48:

Episternum \Ep`i*ster"num\, n.; pl. Episterna. [NL. See
   Epi-, and Sternum.]
   1. (Anat.)
      (a) A median bone connected with the sternum, in many
          vertebrates; the interclavicle.
      (b) Same as Epiplastron.
          [1913 Webster]

   2. (Zo["o]l.) One of the lateral pieces next to the sternum
      in the thorax of insects.
      [1913 Webster]