The principal deck of a large ship or vessel.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "main deck" in a sentence
"Decks below the main deck do not have the initial zero, and are numbered down from the main deck; e.g. Deck 11 is below Deck 3."
"The lower deck guns of the German armored cruisers were able to fire, but the main deck casements of Good Hope and Monmouth had to be kept closed lest the guns be smothered by the sea."
"Scotty stood on the main deck of the Neptune, staring into a two-hundred-liter fish tank."