orifice, porta, opening - an aperture or hole opening into a bodily cavity; "the orifice into the aorta from the lower left chamber of the heart"

rectum - the terminal section of the alimentary canal; from the sigmoid flexure to the anus

imperforate anus - a congenital defect of the anus; there is partial or complete obstruction of the anal opening

anal sphincter, musculus sphincter ani, sphincter ani - the sphincter muscle of the anus