1. (acting) A person who shares star billing.
- The once famous actor objected to his costar having a bigger dressing room.
2. (acting) A person who slightly lacks the status to be considered a star.
- Alas, always a costar but never a star.
1. 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.