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]