The Collaborative International Dictionary of English v.0.48:

Placenta \Pla*cen"ta\, n.; pl. L. Placent[ae], E. Placentas.
   [L., a cake, Gr. ? a flat cake, from ? flat, fr. ?, ?,
   anything flat and broad.]
   1. (Anat.) The vascular appendage which connects the fetus
      with the parent, and is cast off in parturition with the
      afterbirth.
      [1913 Webster]

   Note: In most mammals the placenta is principally developed
         from the allantois and chorion, and tufts of vascular
         villi on its surface penetrate the blood vessels of the
         parental uterus, and thus establish a nutritive and
         excretory connection between the blood of the fetus and
         that of the parent, though the blood itself does not
         flow from one to the other.
         [1913 Webster]

   2. (Bot.) The part of a pistil or fruit to which the ovules
      or seeds are attached.
      [1913 Webster]

WordNet (r) 3.0 (2006):

placenta
    n 1: that part of the ovary of a flowering plant where the
         ovules form
    2: the vascular structure in the uterus of most mammals
       providing oxygen and nutrients for and transferring wastes
       from the developing fetus