pre·cur·sor / ˈprēˌkərsər; priˈkər-/ • n. a person or thing that comes before another of the same kind; a forerunner: a three-stringed precursor of the violin| [as adj.] precursor cells. ∎ Biochem. a substance from which another is formed, esp. by metabolic reaction: pepsinogen is the inactive precursor of pepsin.