Ostium

·noun An opening; a passage.

ostium
ostĭum, ii, n. [kindred with Sanscr. oshtha, labium; Slav. ūsta, the same; cf. os], a door (class.; ...

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.
ostium
ōstium ī, n cf. os, a door : extra ostium, out of doors , T.: aperire, open , T.: operire, shut...

An Elementary Latin Dictionary