Bill of Attainder

In English common law, a bill of attainder, passed by Parliament, was sometimes used to strip a person of his rights and seize his property in the case of serious crimes like treason.  The Americans also used this method to deprive "loyalists" – people who were loyal to Great Britain during the American Revolution – of their property.

By banning the practice the Framers are making it clear that only the courts can judge a person guilty of a crime.  This is not the role of the legislature under the separation of powers.