1) Nautical terminology 2) The undersurface of a deck 
A deckhead is the underside of a deck in a ship. It bears the same relationship to a compartment on the deck below as does the ceiling to the room of a house. ...
The under-side of the deck above. Sometimes paneled over to hide the pipe work. This paneling, like that lining the bottom and sides of the holds, is the ceiling.
The under-side of the deck above. Sometimes paneled over to hide the pipe work. This paneling, like that lining the bottom and sides of the holds, is the ceiling.
The underside of the deck, the cabin ceiling.
The under-side of the deck above. Sometimes paneled over to hide the pipe work. This paneling, like that lining the bottom and sides of the holds, is the ceiling.
the undersurface of a deck.