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]