second deck vs main deck

second deck and main deck both are nouns.

Word	Noun	Adjective	Verb	Adverb
second deck	Yes	No	No	No
main deck	Yes	No	No	No

As nouns, second deck and main deck are synonyms defined as:

second deck and main deck: the uppermost sheltered deck that runs the entire length of a large vessel

second deck (noun)	main deck (noun)
the uppermost sheltered deck that runs the entire length of a large vessel	the uppermost sheltered deck that runs the entire length of a large vessel