What is Amercement?

Legal Definition
An amercement is a financial penalty in English law, common during the Middle Ages, imposed either by the court or by peers. The noun "amercement" lately derives from the verb to amerce, thus: the King amerces his subject, who offended some law. The term is of Anglo-Norman origin (Law French, from French, from Latin), and literally means "being at the mercy of": a-merce-ment (English mercy is cognate).

While it is often synonymous with a fine, it differs in that a fine is a fixed sum prescribed by statute and was often voluntary, while an amercement is arbitrary. Amercements were commonly used as a punishment for minor offenses (such as trespassing in the King's forest), as an alternative to imprisonment.
-- Wikipedia
Legal Definition
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, 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or orders of the court, he might be amerced; that is, a penalty might be imposed upon bim; but this practice has been superseded by attachment.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.
-- Bouviers Law Dictionary
Legal Definition
A pecuniary penalty, in the nature of a fine, imposed upon a person for some fault or misconduct, he being "in mercy" for his offence. It was assessed by the peers of the delinquent or the af-feerors or imposed arbitrarily at the discretion of the court or the lord. Goodyear v. Sawyer (C. C.) 17 Fed. 9.

The difference between amercements and fines is as follows: The latter are certain, and are created by some statute; they can only be imposed and assessed by courts of record; the former are arbitrarily imposed by courts not of record, as courts-leet. Termes de la Ley, 40.

The word "amercement" has long been especially used of a mulct or penalty, imposed by a court upon its own officers for neglect of duty, of failure to pay over moneys collected. In particular, the remedy against a sheriff for failing to levy an execution or make return of proceeds of sale is, in several of the states, known as "amercement" In others, the same result Is reached by process of attachment Abbott. Stansbury v. Mfg. Ch, 5 N. J. Law, 441.
-- Black's Law Dictionary
Legal Definition
A punishment imposed by the court upon an unsuccessful plaintiff for making a false claim. See 3 Bl. Comm. 376.
-- Ballentine's Law Dictionary