topgallant mast vs topgallant

topgallant mast and topgallant both are nouns.

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

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

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

topgallant mast (noun)	topgallant (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