Source : Webster's Revised Unabridged Dictionary (1913)

Decidua \De*cid"u*a\ (?; 135), n. [NL., fr. L. deciduus. See
   {Deciduous}.] (Anat.)
   The inner layer of the wall of the uterus, which envelops the
   embryo, forms a part of the placenta, and is discharged with
   it.

Source : WordNet®

decidua
     n : the epithelial tissue of the endometrium
     [also: {deciduae} (pl)]