Noun
formerly any deck other than the weather deck having cannons from end to end
Source: WordNetOn 16 September, at 11:25am, U-156, with a Red Cross flag draped across her gun deck, was spotted by an American B-24 Liberator bomber from Ascension Island. Source: Internet
The first ironclads were frigates, in the sense of having one gun deck; however, they were also clearly the most powerful ships in the navy, and were principally to serve in the line of battle. Source: Internet