The Collaborative International Dictionary of English v.0.48:

Plagiarist \Pla"gia*rist\, n.
   One who plagiarizes, or purloins the words, writings, or
   ideas of another, and passes them off as his own; a literary
   thief; a plagiary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plagiarist
    n 1: someone who uses another person's words or ideas as if they
         were his own [syn: plagiarist, plagiarizer,
         plagiariser, literary pirate, pirate]