Mate

Mate is an assistant, a deputy, or a second in any work. In the navy the use of the term is now confined to petty officers, such as boatswain's-mate, gunner's-mate, &c. In the merchant-service the mates are important officers, holding functions not greatly inferior to those of lieutenants in the royal navy. The first mate ranks next to the master or captain, commands in his absence, and is immediately responsible for the state of the vessel; the second and third (and fourth in large vessels) have various analogous duties.