Plain English Meaning
A swollen, irritated condition inside the hollow spaces in the face and forehead, often caused by a cold or infection, that can block the small passages those spaces use to drain.
Full Definition
Inflammation of the lining of one or more of the sinuses (the air-filled cavities in the bones of the face and skull). The inflammation causes swelling that can block the small openings connecting each sinus to the nasal passages, trapping air and mucus inside.
Context Anchor
Seen in aeromedical discussions about colds, allergies, sinus blockage, and whether a pilot is fit to fly when pressure changes are expected.
Derivation
From the Latin sinus meaning a curve, fold, or hollow, plus the medical suffix -itis meaning inflammation. So sinusitis literally means inflammation of the hollow spaces.
Why Pilots Care
Swollen sinuses trap air that cannot equalize, producing pain, possible tissue damage, or distraction that compromises safety during altitude changes.