Definition from Wiktionary, the free dictionary
two-decker (plural two-deckers)
- A sail warship, which carried her guns on two fully-armed decks. Usually additional guns were carried on the upper works (forecastle and quarterdeck) but this was not a continuous battery so were not counted. Two-deckers ranged all the way from the small 40-gun fourth-rate up to 80- or even 90-gun ships, with the third-rate or "seventy-four" being the archetype.