The Collaborative International Dictionary of English v.0.48:

Champerty \Cham"per*ty\, n. [F. champart field rent, L.
   campipars; champ (L. campus) field + part (L. pars) share.]
   1. Partnership in power; equal share of authority. [Obs.]
      [1913 Webster]

            Beaut['e] ne sleighte, strengthe ne hardyness,
            Ne may with Venus holde champartye.   --Chaucer.
      [1913 Webster]

   2. (Law) The prosecution or defense of a suit, whether by
      furnishing money or personal services, by one who has no
      legitimate concern therein, in consideration of an
      agreement that he shall receive, in the event of success,
      a share of the matter in suit; maintenance with the
      addition of an agreement to divide the thing in suit. See
      Maintenance.
      [1913 Webster]

   Note: By many authorities champerty is defined as an
         agreement of this nature. From early times the offence
         of champerty has been forbidden and punishable.
         [1913 Webster]

WordNet (r) 3.0 (2006):

champerty
    n 1: an unethical agreement between an attorney and client that
         the attorney would sue and pay the costs of the client's
         suit in return for a portion of the damages awarded;
         "soliciting personal injury cases may constitute champerty"

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

CHAMPERTY, crimes. A bargain with a plaintiff or defendant, campum partire,
to divide the land or other matter sued for between them, if they prevail at
law, the champertor undertaking to carry on the suit at his own expense. 1
Pick. 416; 1 Ham. 132; 5 Monr. 416; 4 Litt. 117; 5 John. Ch. R. 44; 7 Port.
R. 488.
     2. This offence differs from maintenance, in this, that in the latter
the person assisting the suitor receives no benefit, while in the former he
receives one half, or other portion, of the thing sued for. See Punishment;
Fine; Imprisonment; 4 Bl. Com. 135.
     3. This was an offence in the civil law. Poth. Pand. lib. 3, t. 1; App.
n. 1, tom. 3, p. 104; 15 Ves. 139; 7 Bligh's R. 369; S. C. 20 E. C. L. R.
165; 5 Moore & P. 193; 6 Carr. & P. 749; S. C. 25 E. C. L. R. 631; 1 Russ.
Cr. 179 Hawk. P. C. b. 1 c. 84, s. 5.
     4. To maintain a defendant may be champerty. Hawk. P. C. b. 1, c. 84, s.

8 3 Ham. 541; 6 Monr. 392; 8 Yerg. 484; 8 John. 479; 1 John. Ch. R. 444;, 7
Wend. 152; 3 Cowen, 624; 6 Cowen, 90.