atre·sia \ə.ˈtrēzh(ē)ə\noun (-s) Etymology: New Latin, from a- (II) + -tresia 1.medicine: absence or closure of a natural passage of the body : imperforation < atresia of the small intestine > 2.medicine: involution of a part (as of an ovarian follicle not destined to produce a functional ovum)