an act of legislature finding a person guilty of treason or felony without trial.
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British Dictionary definitions for bill of attainder
(formerly) a legislative act finding a person guilty without trial of treason or felony and declaring him attainted: See also attainder (def. 1)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012