Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

endometrium
     n : (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
     [also: {endometria} (pl)]