co·star / ˈkōˌstär; kōˈstär/ (also co-star) • n. a leading actor or actress appearing in a movie, on stage, etc., with another or others of equal importance.• v. [intr.] appear in a production as a costar: she costarred with Robert De Niro in the movie version. ∎ [tr.] (of a production) include as a costar: his latest movie costars Meryl Streep.