The Collaborative International Dictionary of English v.0.48:

Plagiarize \Pla"gia*rize\, v. t. [imp. & p. p. Plagiarized; p.
   pr. & vb. n. Plagiarizing.]
   To steal or purloin from the writings of another; to
   appropriate without due acknowledgement (the ideas or
   expressions of another).
   [1913 Webster]

WordNet (r) 3.0 (2006):

plagiarize
    v 1: take without referencing from someone else's writing or
         speech; of intellectual property [syn: plagiarize,
         plagiarise, lift]

The Devil's Dictionary (1881-1906):

PLAGIARIZE, v.  To take the thought or style of another writer whom
one has never, never read.