What is Bill Of Attainder?

Legal Definition
A bill of attainder (also known as an act of attainder or writ of attainder or bill of pains and penalties) is an act of a legislature declaring a person or group of persons guilty of some crime and punishing them, often without a trial. As with attainder resulting from the normal judicial process, the effect of such a bill is to nullify the targeted person's civil rights, most notably the right to own property (and thus pass it on to heirs), the right to a title of nobility, and, in at least the original usage, the right to life itself. Bills of attainder passed in Parliament by Henry Vlll on the 29 January 1542 resulted in the executions of a number of notable historical figures.

The use of these bills by Parliament eventually fell into disfavor due to the obvious potential for abuse and the violation of several legal principles, most importantly the separation of powers, the right to due process, and the precept that a law should address a particular form of behaviour rather than a specific individual or group. For these reasons, bills of attainder are expressly banned by Article I, section 9, of the United States Constitution (1787) as well as by the constitutions of all 50 US states.
-- Wikipedia
Legal Definition
When a legislative act declares a particular person guilty of a crime (usually treason), such bills are prohibited under Article I, Section 9 of the Constitution.
Legal Definition
Legislation, punishment. An act of the legislature by which one or more persons are declared to be attainted, and their property confiscated.

2. The Constitution of the United States declares that no state shall pass any bill of attainder.

3. During the revolutionary war, bills of attainder, and ox post facto acts of confiscation, were passed to a wide extent. The evils resulting from them, in times of more cool reflection, were discovered to have far outweighed any imagined good. Story on Const. §1367. Vide Attainder; Bill of Pains and Penalties.
-- Bouviers Law Dictionary
Legal Definition
A legislative act, directed against a designated person, pronouncing him guilty of an alleged crime, (usually treason,) without trial or conviction according to the recognized rules of procedure, and passing sentence of death and attainder upon him. "Bills of attainder," as they are technically called, are such special acts of the legislature as inflict capital punishments upon persons supposed to be guilty of high offenses, such as trCason and felony, without any conviction in the ordinary course of judicial proceedings If an act inflicts a milder degree of punishment than death, it is called a "bili of pains and penalties," but bath are included in the prohibition in the Federal constitution. Story, Const. § 1344; Cummings v. Missouri, 4 Wall. 323, 18 In Ed. 356; Ex parte Garland, 4 Wall. 387, 18 In Ed. 366; People v. Hayes, 140 N. Y. 484, 35 N. E. 951, 23 L. R. A. 830, 37 Am. St. Ren. 572; Green v. Shumway, 39 N. Y. 431; In re Yung Sing Hee (C. C.) 36 Fed. 439.
-- Black's Law Dictionary
Legal Definition
A statute attainting a person. See 18 L. Ed. (U. S.) 366.
-- Ballentine's Law Dictionary