1) The process of charging a public official, such as the U.S. president or a federal judge, with a crime or misconduct, which results in a trial by the senate to determine whether the official should be removed from office.  See separate article entitled "Impeachment."

2) To discredit, for example, to show that a witness is not believable--perhaps because the witness made statements that are inconsistent with present testimony, or has a reputation for not being a truthful person.  See separate article entitled "Impeachment of a Witness."