Definition - What does Main Deck mean?
The Main Deck is the topmost complete deck that extends from the bow to the stern. The steel hull of the ship is considered as the structural beam with keel that forms the lower strength member and with Main Deck that forms the upper flange of a box girder. It is the principle deck of the vessel and the uppermost deck of the hull in some of the ships. It is usually considered as the weather deck, but not always. It is often the deck present under the upper deck in the sailing warships.
Petropedia explains Main Deck
The Main Deck could be the highest continuous deck which is so constructed to make it possible to securely close all openings. It can act as the tension member of the ship when it is supported by only single wave amidships or is considered as the compression member when it is supported between the aft and the waves forward. It can also be the deck below the shelter or the upper deck of the merchant ship generally found at the transverse framing's end.