NOUN (1)

1. one of two actors who are given equal status as stars in a play or film;


VERB (2)

1. feature as the co-star in a performance;

2. be the co-star in a performance;

The Collaborative International Dictionary of English v.0.48:

co-star \co-star\ v. t.
   to feature as the co-star in a performance; as, the film
   co-starred Robin Williams as the psychiatrist.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

co-star \co-star\ v. i.
   to be the co-star in a performance; as, Robin Williams
   co-starred in the film.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

co-star
    n 1: one of two actors who are given equal status as stars in a
         play or film
    v 1: feature as the co-star in a performance
    2: be the co-star in a performance