Ostium

An ostium is a small opening or orifice, as in a body organ or passage.

It can mean the following
 * Any of the small openings or pores in a sponge.

Anatomy

 * Ostium of Fallopian tube.
 * Ostium primum or foramen ovale (ostium secundum) of the developing heart.
 * Ostium maxillare of the Maxillary Sinus