1 congenital absence or abnormal narrowing of a body opening. See biliary atresia, duodenal atresia, tricuspid atresia.
2 the degenerative process that affects the majority of ovarian follicles. Usually only one Graafian follicle will ovulate in each menstrual cycle.
—atretic adj.

an atrium, especially the atrium of the heart. Example: atrioventricular (relating to the atria and ventricles of the heart).