Nasal passage

Sinus ostium
Details
Latin	Sinus ostium (plural: sinus ostia)
Anatomical terminology []

A sinus ostium is the opening that connects a sinus to the nasal cavity itself. It is a tight area that tends to have a higher percentage of cilia than the surrounding mucosa.

If the sinus ostium is blocked this will cause an accumulation of fluid in the sinus.

...
Wikipedia