- a round metal cover placed over a ship's porthole in stormy weather
- a window of heavy glass set in the deck or side of a ship
- a skylight made so as not to be opened
See deadlight in American Heritage Dictionary 4
a. A strong shutter or plate fastened over a ship's porthole or cabin window in stormy weather.
b. A thick window set in a ship's side or deck.
- A skylight constructed so that it cannot be opened.