topgallant vs topgallant mast

topgallant and topgallant mast both are nouns.

Word	Noun	Adjective	Verb	Adverb
topgallant	Yes	No	No	No
topgallant mast	Yes	No	No	No

As nouns, topgallant and topgallant mast are synonyms defined as:

topgallant and topgallant mast: a mast fixed to the head of a topmast on a square-rigged vessel

topgallant (noun)	topgallant mast (noun)
a mast fixed to the head of a topmast on a square-rigged vessel	a mast fixed to the head of a topmast on a square-rigged vessel
a sail set on a yard of a topgallant mast