ostium ostium an ostium (plural ostia) in anatomy is a small opening or orifice. look up ostium in wiktionary, the free dictionary. ostium or ostia may refer to: human anatomy ostium of fallopian tube ostium of the uterus (disambiguation) ostium primum of the developing heart ostium secundum (foramen ovale) of the developing heart ostium maxillare of the maxillary sinus ostium vaginae (vaginal orifice) coronary ostium – opening of coronary arteries at root of aorta, superior to aortic valve sinus ostium an opening that connects a sinus to the nasal cavity itself. in other animals ostium (sponges), the pores of living sponges ostia of female lepidoptera genitalia