The Collaborative International Dictionary of English v.0.48:

Attainder \At*tain"der\, n. [OF. ataindre, ateindre, to accuse,
   convict. Attainder is often erroneously referred to F.
   teindre tie stain. See Attaint, Attain.]
   1. The act of attainting, or the state of being attainted;
      the extinction of the civil rights and capacities of a
      person, consequent upon sentence of death or outlawry; as,
      an act of attainder. --Abbott.
      [1913 Webster]

   Note: Formerly attainder was the inseparable consequence of a
         judicial or legislative sentence for treason or felony,
         and involved the forfeiture of all the real and
         personal property of the condemned person, and such
         "corruption of blood" that he could neither receive nor
         transmit by inheritance, nor could he sue or testify in
         any court, or claim any legal protection or rights. In
         England attainders are now abolished, and in the United
         States the Constitution provides that no bill of
         attainder shall be passed; and no attainder of treason
         (in consequence of a judicial sentence) shall work
         corruption of blood or forfeiture, except during the
         life of the person attainted.
         [1913 Webster]

   2. A stain or staining; state of being in dishonor or
      condemnation. [Obs.]
      [1913 Webster]

            He lived from all attainder of suspect. --Shak.
      [1913 Webster]

   Bill of attainder, a bill brought into, or passed by, a
      legislative body, condemning a person to death or
      outlawry, and attainder, without judicial sentence.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bill of attainder
    n 1: a legislative act finding a person guilty of treason or
         felony without a trial; "bills of attainder are prohibited
         by the Constitution of the United States"

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BILL OF ATTAINDER, 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. Sec. 1367. Vide Attainder;
Bill of Pains and Penalties.