A more thorough explanation:

A bill of attainder is a type of law that declares a person or group guilty of a crime without a trial. This means that the government can punish someone without giving them a fair chance to defend themselves in court. In the United States, bills of attainder are not allowed because they violate the Constitution.

For example, imagine that the government passes a law that says John Smith is guilty of stealing a car. Even though John has not been to court and found guilty, he is now considered a criminal and can be punished. This is not fair because John did not have a chance to defend himself in court.

The Constitution says that bills of attainder are not allowed because they go against the idea of separation of powers. This means that the government cannot take on the role of the courts and punish people without a fair trial.

To determine if a law is a bill of attainder, courts use a three-part test:

1. The law inflicts punishment.
2. The law targets specific named or identifiable individuals or groups.
3. Those individuals or groups would otherwise have judicial protections.

For example, if a law says that all people with the last name Smith are not allowed to vote, this would be a bill of attainder because it targets a specific group of people and takes away their right to vote without a fair trial.