Technical term used in aviation and meteorology for a sudden downdraft or region of low air density that causes an aircraft to drop. Not a vacuum in the strict physical sense.
The plane hit an air pocket and suddenly lost altitude.
Compound of 真空 (vacuum) and 帯 (zone, belt), literally 'vacuum zone'. The term is a calque from English 'air pocket', though the Japanese word misleadingly suggests a true vacuum.