Plain English Meaning
A small hollow space in the bone above your eyebrows that normally lets air flow in and out through your nose. If a cold or congestion blocks that opening, the trapped air can't adjust to changing cabin pressure, which causes sharp forehead pain when you climb or descend.
Full Definition
An air-filled cavity in the bone of the forehead, located just above the eyebrows, that connects to the nasal passage through a small drainage opening. As one of the paranasal sinuses, it equalizes with outside air pressure during normal breathing, but if its drainage opening is blocked by congestion or inflammation, trapped air cannot expand or compress freely as cabin altitude changes, causing pain during climbs and especially during descents.
Context Anchor
Pilots encounter this term in discussions of sinus pain, colds, allergies, and pressure changes during climbs and descents.