The Collaborative International Dictionary of English v.0.48:

Plagiarism \Pla"gia*rism\ (pl[=a]"j[.a]*r[i^]z'm or
   pl[=a]"j[i^]*[.a]*r[i^]z'm; 277), n. [Cf. F. plagiarisme.]
   1. The act or practice of plagiarizing.
      [1913 Webster]

   2. That which is plagiarized; a work which has been
      plagiarized.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plagiarism
    n 1: a piece of writing that has been copied from someone else
         and is presented as being your own work
    2: the act of plagiarizing; taking someone's words or ideas as
       if they were your own [syn: plagiarism, plagiarization,
       plagiarisation, piracy]

The Devil's Dictionary (1881-1906):

PLAGIARISM, n.  A literary coincidence compounded of a discreditable
priority and an honorable subsequence.