1) to discredit the testimony of a witness by proving that he/she has not told the truth or has been inconsistent, by introducing contrary evidence, including statements made outside of the courtroom in depositions or in statements of the witness heard by another. 2) to charge a public official with a public crime for which the punishment is removal from office. one president, andrew johnson in 1868, was charged with violation of federal laws in a politically motivated impeachment, but was acquitted by the margin of one vote in a trial held by the senate. president richard nixon resigned in 1974 rather than face impending impeachment charges brought by the house of representatives in the watergate affair, in which he was accused of obstructing the investigation and lying to congress about his participation. several federal judges have been impeached and nine have been found guilty by the senate.