Seaman, a black Newfoundland dog, was purchased for $20 by Captain Meriwether Lewis for his famed Lewis and Clark expedition. The dog was once thought to have been named Scannon. The matter has been debated often over the years.
One whose occupation is to assist in the management of ships at sea; a mariner; a sailor; -- applied both to officers and common mariners, but especially to the latter. Opposed to landman, or landsman.
Seaman can be a generic term for male sailor. ---- Seaman as a rate refers to the one of the lowest rates in a navy. In Commonwealth context it refers to the lowest rank in the navy, followed by Able Seaman and Leading Seaman, and followed by the Petty Officer ranks.