Other forms: amercements
An amercement is a fine that's imposed by a court of law. During the Middle Ages, you might receive an amercement if you were caught trespassing on the king's property.
Amercement comes from the Anglo-French amerciment and its Old French root a merci, or "to be at the mercy of." The idea is that an amercement is imposed on someone who is at the mercy of the court's decision. These days the word describes a fine that isn't a fixed amount, but is decided by a judge based on the specific offense that's been committed.