Impeachment
Definition
Impeachment is the process of attacking a witness's credibility. Under the Federal Rules of Evidence, a witness may be impeached by showing bias, prior inconsistent statements (FRE 613), character for untruthfulness (FRE 608), prior felony convictions (FRE 609), contradictory evidence, or defects in perception or memory. A party may impeach its own witness under FRE 607. Impeachment evidence is generally admissible only to attack credibility, not as substantive proof, though prior inconsistent statements made under oath may be substantive evidence under FRE 801(d)(1)(A).
Example
A witness testifies that the light was green. On cross-examination, the attorney confronts them with a prior statement: "I told the officer the light was red." This impeaches the witness with a prior inconsistent statement.