Wiktionary
n. 1 (context countable nautical English) The deck of a ship immediately above the hold 2 (context uncountable nautical by extension English) Those members of a ship's company who are not officers.
WordNet
n. the deck below the main deck [syn: third deck]
Usage examples of "lower deck".
The companionway was full of smoke from the fire on the lower deck, but because the air wasn't circulating the conditions weren't as bad as I'd thought they would be.
When, at last, one leg floundered over the side, Maia rolled onto the ship's lower deck and quickly sought shelter behind the mainmast, panting silently with a wide-open mouth, waiting for the pain to dull.
When I came up from the lower deck and you both turned in the darkness, for a moment I couldn't tell which of you was which.
The berth was a screened-off space of the lower deck, on the larboard side, abreast of the main hatch.
Today we boarded the Maidens Chance and jostled companionably for space on the lower deck allotted to us.
Smoke trailed out, then billowed out, from the open gunports all along her lower deck, and then flames belched after it, brilliant orange in the twilight.
Her superstructure and the broad horizontal stripe below her lower deck were painted glacier white.