topgallant yard vs top-gallant yard

topgallant yard and top-gallant yard both are nouns.

Word	Noun	Adjective	Verb	Adverb
topgallant yard	Yes	No	No	No
top-gallant yard	Yes	No	No	No

As nouns, topgallant yard and top-gallant yard are synonyms defined as:

topgallant yard and top-gallant yard: a yard on which a topgallant sail is carried, above the topsail or topsails.

topgallant yard (noun)	top-gallant yard (noun)
a yard on which a topgallant sail is carried, above the topsail or topsails.	a yard on which a topgallant sail is carried, above the topsail or topsails.