The Collaborative International Dictionary of English v.0.48:

Top billing \Top" bil*ling\, n.
   the most prominent place in an advertisement or program for a
   theatrical performance, movie, fundraising affair, etc.; the
   place where the star's name usually is located.
   [PJC]

WordNet (r) 3.0 (2006):

top billing
    n 1: the advertisement of a star's name at the top of a
         theatrical poster