seaman vs Jack

seaman and Jack both are nouns.

Word	Noun	Adjective	Verb	Adverb
seaman	Yes	No	No	No
Jack	Yes	No	No	No

As nouns, seaman and Jack are synonyms defined as:

Other synonyms of seaman include gob, Jack-tar, mariner, old salt, sea dog, seafarer, tar.

seaman (noun)	Jack (noun)
a man who serves as a sailor	a man who serves as a sailor