Also called chief officer.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
Examples from the Web for chief mate
"Yes," rejoined a Guernsey-man from the bulwarks, who turned out to be the chief-mate.Moby Dick; or The Whale
Just as the sun rose, the captain and chief-mate made their appearance on deck.
Marble insisted on taking Talcott's place, and he now became my chief-mate, as I had once been his.
“Sounds as though there was something wrong aboard the barque yonder, sir,” reported one of the men to the chief-mate.
Mr Bryce, the chief-mate of the Flying Cloud, was one of those unfortunate men who are always more or less in an ill humour.