costar /ˈkəʊstɑː(ɹ)/noun 2verb 2forms 3 noun2 •A person who shares star billing with another.The once famous actor objected to his costar having a bigger dressing room. •A person who slightly lacks the status to be considered a star.Alas, always a costar but never a star. verb2 •To perform with the billing of a costar.People thought his career was over but now he will get to costar on Broadway next month. •To feature (a number of star performers) jointly. formscostarred · costarring · costars