The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
decidua n 1: the epithelial tissue of the endometrium