topmast vs royal mast

topmast and royal mast both are nouns.

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

As nouns, royal mast is a hyponym of topmast; that is, royal mast is a word with a more specific, narrower meaning than topmast:

Other hyponyms of topmast include fore-topmast, main-topmast, topgallant, topgallant mast, mizzen-topmast.

topmast (noun)	royal mast (noun)
the mast next above a lower mast and topmost in a fore-and-aft rig	topmast immediately above the topgallant mast