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]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "plagiarism":
   adoption, appropriation, assumption, autoplagiarism,
   borrowed plumes, borrowing, copying, counterfeiting, cribbing,
   derivation, deriving, doubling, duplication, echo, emulation,
   fakery, following, forgery, hit-off, imitation, impersonation,
   imposture, impression, infringement, infringement of copyright,
   infringing, lifting, literary piracy, mimesis, mirroring, mocking,
   onomatopoeia, parody, pasticcio, pastiche, piracy, pirating,
   plagiarizing, plagiary, purloining, quotation, reappearance,
   rebirth, recurrence, redoubling, reduplication, reecho,
   regurgitation, reincarnation, renewal, reoccurrence, repetition,
   reproduction, resumption, return, simulation, stealing, takeoff,
   taking, theft, thievery, usurpation



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLAGIARISM. The act of appropriating the ideas and language of another, and
passing them for one's own.
     2. When this amounts to piracy the party who has been guilty of it will
be enjoined, when the original author has a copyright. Vide Copyright;
Piracy; Quotation; Pard. Dr. Com. n. 169.



The Devil's Dictionary (1881-1906):

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