The Collaborative International Dictionary of English v.0.48:

Authorship \Au"thor*ship\, n.
   1. The quality or state of being an author; function or
      dignity of an author.
      [1913 Webster]

   2. Source; origin; origination; as, the authorship of a book
      or review, or of an act, or state of affairs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

authorship
    n 1: the act of creating written works; "writing was a form of
         therapy for him"; "it was a matter of disputed authorship"
         [syn: writing, authorship, composition, penning]
    2: the act of initiating a new idea or theory or writing; "the
       authorship of the theory is disputed" [syn: authorship,
       paternity]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "authorship":
   artistry, authorcraft, automatic writing, beginning,
   cacoethes scribendi, coinage, composition, conception, concoction,
   contrivance, contriving, creation, creative effort,
   creative writing, devising, drama-writing, editorial-writing,
   essay-writing, expository writing, fabrication,
   facility in writing, feature-writing, generation, graphomania,
   graphorrhea, graphospasm, hatching, improvisation, inditement,
   invention, journalism, libretto-writing, literary artistry,
   literary composition, literary power, literary production,
   literary talent, making do, mintage, novel-writing, origination,
   pen, pencraft, playwriting, ready pen, rewriting,
   short-story writing, skill with words, technical writing,
   verse-writing, writing