A sinus is a cavity, channel, or space within the body, often referring to one of the air-filled spaces connected to the nasal passages.
In everyday medical use, sinus commonly refers to a paranasal sinus, one of the air-filled cavities within the bones around the nose. There are four paired groups: the frontal, maxillary, ethmoid, and sphenoid sinuses. They are lined with mucous membrane and connect with the nasal cavity.
The word sinus can also have other anatomical meanings. More broadly, it can describe a cavity or channel in the body, so its precise meaning depends on context.
QUICK FACT
The sinuses are not simply empty holes in the skull. They are lined with mucous membrane and communicate with the nasal cavity.
COMMONLY CONFUSED WITH
Nasal cavity: The nasal cavity is the main passage through the nose; the paranasal sinuses are separate air-filled spaces that connect with it.
SEE ALSO