(in the Middle Ages) a man who served his sovereign or lord as a mounted soldier in armor. (in the UK) a man awarded a nonhereditary title by the sovereign in recognition of merit or service and entitled to use the honorific “Sir” in front of his name. A chess piece, typically with its top shaped like a horse's head, that moves by jumping to the opposite corner of a rectangle two squares by three. Invest (someone) with the title of knight.