Bill of Attainder is a legislative act pronouncing a person guilty of a crime, and punishing them without benefit of a trial. This usually done is cases of treason.
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." "The Bill of Attainder Clause was intended not as a narrow, technical (and therefore soon to be outmoded) prohibition, but rather as an implementation of the separation of powers, a general safeguard against legislative exercise of the judicial function or more simply - trial by legislature." U.S. v. Brown, 381 U.S. 437, 440 (1965).