The Collaborative International Dictionary of English v.0.48:

Plowshare \Plow"share`\, Ploughshare \Plough"share"\, n.
   The share of a plow, or that part which cuts the slice of
   earth or sod at the bottom of the furrow.
   [1913 Webster]

   Plowshare bone (Anat.), the pygostyle.
      [1913 Webster] Plowtail

WordNet (r) 3.0 (2006):

ploughshare
    n 1: a sharp steel wedge that cuts loose the top layer of soil
         [syn: plowshare, ploughshare, share]