topgallant mast vs topmast

topgallant mast and topmast both are nouns.

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

As nouns, topmast is a hypernym of topgallant mast; that is, topmast is a word with a broader meaning than topgallant mast:

topgallant mast: a mast fixed to the head of a topmast on a square-rigged vessel
topmast: the mast next above a lower mast and topmost in a fore-and-aft rig

topgallant mast (noun)	topmast (noun)
a mast fixed to the head of a topmast on a square-rigged vessel	the mast next above a lower mast and topmost in a fore-and-aft rig