An amercement is a financial penalty in English law, common during the Middle Ages, imposed either by the court or by peers. 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. They were commonly used as a punishment for minor offenses (such as trespassing in the King's forest), as an alternative to imprisonment.
Amercements are much mentioned in Magna Carta, particularly article 20:
"A free man shall not be amerced for a trivial offence except in accordance with the degree of the offence, and for a grave offence he shall be amerced in accordance with its gravity, yet saving his way of living; and a merchant in the same way, saving his stock-in-trade; and a villein shall be amerced in the same way, saving his means of livelihood--if they have fallen into our mercy: and none of the aforesaid amercements shall be imposed except by the oath of good men of the neighbourhood."
Referred to in Frantz v. U.S. Powerlifting Federation 836 F.2d 1063 (7th Cir. 1987). In a discussion about the imposition of FRCP Rule 11 sanctions on a plaintiff's attorney, the decision says, "The complaint in this case was frivolous, which calls at a minimum for censure of Victor D. Quilici, the plaintiffs' lawyer. Whether it calls for amercement - and, if so, whether Cotter or the Treasury is the appropriate beneficiary - is something the district court should consider as an initial matter."
A cause of action in amercement will exist against a sheriff who refused to seize property under a writ of execution. Vitale v. Hotel California, Inc., 446 A.2d 880 (N.J. Super. Ct. Law 1982).