A "bill of attainder" is any act of a legislative body declaring a person or group of persons guilty of a crime and assessing a punishment without the benefit of trial. The Constitution of the United States, Article I, Section 9, paragraph 3 provides that: "No Bill of Attainder or ex post facto Law will be passed."
Bills of attainder were commonly used in England during the 18th century and were applied to the British colonies. Anger over the application of bills attainder in the colonies was one of the motivations for the American Revolution.
As James Madison wrote in, Federalist Number 44, "Bills of attainder, ex post facto laws, and laws impairing the obligations of contracts, are contrary to the first principles of the social compact, and to every principle of sound legislation. ... The sober people of America are weary of the fluctuating policy which has directed the public councils. They have seen with regret and indignation that sudden changes and legislative interferences, in cases affecting personal rights, become jobs in the hands of enterprising and influential speculators, and snares to the more-industrious and less-informed part of the community."