The Collaborative International Dictionary of English v.0.48:

Amerciament \A*mer"cia*ment\, n. [LL. amerciamentum.]
   Same as Amercement. --Mozley & W.
   [1913 Webster]

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]

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

AMERCIAMENT, AMERCEMENT, English law. A pecuniary punishment arbitrarily
imposed by some lord or count, in distinction from a fine which is expressed
according to the statute. Kitch. 78. Amerciament royal, when the amerciament
is made by the sheriff, or any other officer of the king. 4 Bl. Com. 372.