NOUN (3)

1. an exorbitant charge;

2. unjust exaction (as by the misuse of authority);
- Example: "the extortion by dishonest officials of fees for performing their sworn duty"

3. the felonious act of extorting money (as by threats of violence);

The Collaborative International Dictionary of English v.0.48:

Extortion \Ex*tor"tion\, n. [F. extorsion.]
   1. The act of extorting; the act or practice of wresting
      anything from a person by force, by threats, or by any
      undue exercise of power; undue exaction; overcharge.
      [1913 Webster]

   2. (Law) The offense committed by an officer who corruptly
      claims and takes, as his fee, money, or other thing of
      value, that is not due, or more than is due, or before it
      is due. --Abbott.
      [1913 Webster]

   3. That which is extorted or exacted by force.

   Syn: Oppression; rapacity; exaction; overcharge.
        [1913 Webster]

WordNet (r) 3.0 (2006):

extortion
    n 1: an exorbitant charge
    2: unjust exaction (as by the misuse of authority); "the
       extortion by dishonest officials of fees for performing their
       sworn duty"
    3: the felonious act of extorting money (as by threats of
       violence)

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "extortion":
   armed robbery, asportation, assault and robbery, badger game,
   banditry, bank robbery, blackmail, bloodsucking, call, call for,
   cattle lifting, cattle stealing, claim, contribution, demand,
   demand for, draft, drain, duty, exaction, exploitation,
   extortionate demand, heavy demand, heist, highway robbery,
   hijacking, holdup, imposition, impost, indent, insistent demand,
   levy, loan-sharking, mugging, nonnegotiable demand, notice, order,
   overassessment, overcharge, pocket picking, protection racket,
   purse snatching, rending, requirement, requisition, ripping,
   robbery, robbing, rush, rush order, shakedown, shylocking, stickup,
   stickup job, surcharge, tax, taxing, tearing, tribute, ultimatum,
   usury, vampirism, warning, wrench, wrenching, wrest, wresting,
   wring, wringing



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

EXTORTION, crimes. In a large sense it, signifies any oppression, under
color of right: but in a more strict sense it means the unlawful taking by
any officer, by color of his office, of any money or thing of value that is
not due to him, or more than is due, or before it is due. 4 Bl. Com. 141; 1
Hawk. P. C. c. 68, s. 1; 1 Russ. Cr. *144. To constitute extortion, there
must be the receipt of money or something of value; the taking a promissory
note, which is void, is. not sufficient to make an extortion. 2 Mass. R.
523; see Bac. Ab. h.t.; Co. Litt. 168. It is extortion and oppression for
an officer to take money for the performance of his duty, even though it be
in the exercise of a discretionary power. 2 Burr. 927. It differs from
exaction. (q.v.) See 6 Cowen, R. 661; 1 Caines, R. 130; 13 S. & R. 426 1
Yeates, 71; 1 South. 324; 3 Penna. R. 183; 7 Pick.  279; 1 Pick. 171.