Nautical. the clear space between two decks.
2. Also called headway. a.
clear vertical space, as between the head and sill of a doorway, the ceiling and floor of a room, or the ceiling of a vehicular passageway and a vehicle roof, as to allow passage or comfortable occupancy: over 7.5 feet of headroom in the attic; a covered bridge with limited headroom.
clear vertical space above one’s head, as in a vehicle or room: plenty of headroom for passengers.