A two-decker is a sail warship which carried her guns on two fully armed decks. Wikipedia

Sailing ship with 2 decks armed with cannon. Wikipedia Disambiguation

Ship type Wikidata

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 was 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. Wiktionary

Warship. Wiktionary (translation)