frontal sinuses

Definition of frontal sinus
National Cancer Institute

frontal sinus

(FRUN-tul SY-nus)
A type of paranasal sinus (a hollow space in the bones around the nose). There are two, large frontal sinuses in the frontal bone, which forms the lower part of the forehead and reaches over the eye sockets and eyebrows. The frontal sinuses are lined with cells that make mucus to keep the nose from drying out.

Source: NCI Dictionary of Cancer Terms