decidua

This term is used exclusively within medical and anatomical contexts, specifically in obstetrics and gynecology. It describes the specialized layer of the uterine lining that develops during pregnancy to support the embryo and is subsequently expelled during the postpartum period.

Because it refers to a specific biological tissue layer, it is treated as an uncountable noun in clinical descriptions. It is rarely used in general conversation and is typically reserved for pathology reports, medical textbooks, or surgical notes.