The Collaborative International Dictionary of English v.0.48:

Champertor \Cham"per*tor\, n. [F. champarteur a divider of
   fields or field rent. See Champerty.] (Law)
   One guilty of champerty; one who purchases a suit, or the
   right of suing, and carries it on at his own expense, in
   order to obtain a share of the gain.
   [1913 Webster]

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

CHAMPERTOR, crim. law. One who makes pleas or suits, or causes them to be
moved, either directly or indirectly, and sues them at his proper costs,
upon condition of having a part of the gain.