Used for someone whose job involves working on a ship or boat, more professional term than 'sailor'. Mariners are trained to handle all types of sea vessels.
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Literal Use
Mariner should be used for actual seafaring roles, not metaphorically for travellers or adventurers. The mariners prepared for the long voyage ahead.
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Language Evolution
'Mariner' may sound slightly old-fashioned; 'sailor' or similar terms are more common today. The old mariner shared tales from his youth at sea.