Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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