Meaning of “co-star” - Learner’s Dictionary

co-star

verb uk us /kəʊˈstɑːr/ US  /ˈkəʊstɑːr/ (present participle co-starring, past tense and past participle co-starred)
FILM/PLAY [ T ]

If a film, play, etc co-stars two or more famous actors, they are in it.

ACTOR [ I ]

to be in a film, play, etc with another famous actor:

Hugh Grant co-stars with Julia Roberts in 'Notting Hill'.

(Definition of “co-star verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)