Atresia is a condition in which an orifice or passage in the body is (usually abnormally) closed or absent.

Examples of atresia include:

• Aural atresia, a congenital deformity where the ear canal is underdeveloped.
• Biliary atresia, a condition in newborns in which the common bile duct between the liver and the small intestine is blocked or absent.
• Congenital bronchial atresia, a rare congenital abnormality
• Choanal atresia, blockage of the back of the nasal passage, usually by abnormal bony or soft tissue.
• Esophageal atresia, which affects the alimentary tract and causes the esophagus to end before connecting normally to the stomach.
• Follicular atresia, degeneration and resorption of the ovarian follicles.  
• Imperforate anus, malformation of the opening between the rectum and anus.
• Intestinal atresia, malformation of the intestine, usually resulting from a vascular accident in utero.
• Microtia, absence of the ear canal or failure of the canal to be tubular or fully formed (can be related to Microtia, a congenital deformity of the pinna, or outer ear).
• Ovarian follicle atresia, the degeneration and subsequent resorption of one or more immature ovarian follicles.
• Potter sequence, congenital decreased size of the kidney leading to absolutely no functionality of the kidney, usually related to a single kidney.
• Pulmonary atresia, malformation of the pulmonary valve in which the valve orifice fails to develop.
• Renal agenesis, only having one kidney.
• Tricuspid atresia, a form of congenital heart disease whereby there is a complete absence of the tricuspid valve, and consequently an absence of the right atrioventricular connection.
• Vaginal atresia, a congenital occlusion of the vagina or subsequent adhesion of the walls of the vagina, resulting in its occlusion.