Wordnet 3.0
NOUN (2)
1. 
 the formation of the placenta in the uterus; 
2. 
 arrangement of the ovules in the placenta and of the placentas in the ovary; 
The Collaborative International Dictionary of English v.0.48:
Placentation \Plac`en*ta"tion\, n.
   1. (Anat.) The mode of formation of the placenta in different
      animals; as, the placentation of mammals.
      [1913 Webster]
   2. (Bot.) The mode in which the placenta is arranged or
      composed; as, axile placentation; parietal placentation.
      [1913 Webster]
WordNet (r) 3.0 (2006):
placentation
    n 1: the formation of the placenta in the uterus
    2: arrangement of the ovules in the placenta and of the
       placentas in the ovary