Source : Webster's Revised Unabridged Dictionary (1913)

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 r[^o]le of philanthropist.

   {Title r[^o]le}, the part, or character, which gives the
      title to a play, as the part of Hamlet in the play of that
      name.

Source : WordNet®

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