The Collaborative International Dictionary of English v.0.48:

Role \Role\, Role \R[^o]le\, n. [F. See Roll.]
   A part, or character, performed by an actor in a drama;
   hence, a part of function taken or assumed by any one; as, he
   has now taken the role of philanthropist.
   [1913 Webster]

   Title role, the part, or character, which gives the title
      to a play, as the part of Hamlet in the play of that name.
      [1913 Webster]

WordNet (r) 3.0 (2006):

title role
    n 1: the role of the character after whom the play is named
         [syn: title role, name part]