The Collaborative International Dictionary of English v.0.48:

Amercement \A*merce"ment\, n. [OF. amerciment.]
   The infliction of a penalty at the discretion of the court;
   also, a mulct or penalty thus imposed. It differs from a
   fine, in that the latter is, or was originally, a fixed and
   certain sum prescribed by statute for an offense; but an
   amercement is arbitrary. Hence, the act or practice of
   affeering. [See Affeer.] --Blackstone.
   [1913 Webster]

   Note: This word, in old books, is written amerciament.
         [1913 Webster]

   Amercement royal, a penalty imposed on an officer for a
      misdemeanor in his office. --Jacobs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

amercement
    n 1: money extracted as a penalty [syn: fine, mulct,
         amercement]

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

AMERCEMENT, practice. A pecuniary penalty imposed upon a person who is in
misericordia; as, for example, when the defendant se retaxit, or recessit in
contemptum curioe. 8 Co. 58; Bar. Ab. Fines and Amercements. By the common
law, none can be amerced in his absence, except for his default. Non licet
aliquem in sua absentia amerciare nisi per ejus defaltas. Fleta, lib. 2,
cap. 65, Sec. 15.
     2. Formerly, if the sheriff failed in obeying the writs, rules, or
orders of the court, he might be amerced; that is, a penalty might be
imposed upon him; but this practice has been superseded by attachment. In
New Jersey and Ohio, the sheriff may, by statutory provision, be amerced for
making a return contrary to the provision of the statute. Coxe, 136, 169; 6
Halst. 334; 3 Halst. 270, 271; 5 Halst. 319; 1 Green, 159, 341; 2 Green,
350; 2 South. 433; 1 Ham. 275; 2 Ham. 603; 6 Ham. 452; Wright, 720.