Léonin (also Leoninus, Leonius, Leo) (fl. 1150s — d. ? 1201) is the first known significant composer of polyphonic organum. He was probably French, and he probably lived and worked in Paris at the Notre Dame Cathedral, and was the earliest member of the Notre Dame school of polyphony who is known by name. The name Léonin is derived from "Leoninus," which is the Latin diminutive of the name Leo, thus it is likely that Léonin's given French name was Léo.