bosun vs boatswain

bosun and boatswain both are nouns.

Word	Noun	Adjective	Verb	Adverb
bosun	Yes	No	No	No
boatswain	Yes	No	No	No

As nouns, bosun and boatswain are synonyms defined as:

bosun and boatswain: a petty officer on a merchant ship who controls the work of other seamen

Other synonyms of bosun include bo's'n, bo'sun, bos'n.

bosun (noun)	boatswain (noun)
a petty officer on a merchant ship who controls the work of other seamen	a petty officer on a merchant ship who controls the work of other seamen