From Longman Dictionary of Contemporary Englishfirst mateˌfirst ˈmate (also first officer) noun [countable] TTWBOan officer who has the rank just below captain on a ship that is not a military ship
Examples from the Corpus
first mate• This, his first mate, is known as the primary female.• Three times he watched his first mate get blown to bits.• She must be mated again, and the first mate has to guard her or risk his groundwork going to benefit another.• The captain read the funeral service and the first mate raised the plank and tipped the small bundle into the water.• Starbuck and Ahab almost communicate with each other as the first mate pleads with Ahab to repudiate this vengeful mission.• It was like a shipwreck, where the resourceful child passenger becomes the first mate.• Here then the first mate and the captain are diametrically opposed. 