Wordnet 3.0
NOUN (1)
1. 
 (pregnancy) the mucous membrane that lines the uterus; 
 thickens under hormonal control and (if pregnancy does not occur) is shed in menstruation; 
 if pregnancy occurs it is shed along with the placenta at parturition; 
The Collaborative International Dictionary of English v.0.48:
Endometrium \En`do*me"tri*um\, n. [NL., fr. Gr. 'e`ndon within +
   mh`tra the womb.] (Anat.)
   The membrane lining the inner surface of the uterus, or womb.
   [1913 Webster]
WordNet (r) 3.0 (2006):
endometrium
    n 1: (pregnancy) the mucous membrane that lines the uterus;
         thickens under hormonal control and (if pregnancy does not
         occur) is shed in menstruation; if pregnancy occurs it is
         shed along with the placenta at parturition