'Top billing' definitions:

Definition of 'top billing'

(from WordNet)
noun
The advertisement of a star's name at the top of a theatrical poster

Definition of 'Top billing'

From: GCIDE
  • 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]