Impeaching a witness is to call into question her truthfulness, using evidence which shows she is unworthy of belief. A witness can be impeached using a prior inconsistent statement made in an affidavit or in a deposition.
An example of impeachment is when during trial a person testifies inconsistent with what she has stated before. The witness can be impeached using the prior inconsistent statement from the affidavit or deposition to demonstrate to the judge and jury that the individual is not believable. For example, a witness who stated in deposition that the light for the Defendant was definitely red testifies at trial that the light was yellow or that she is unsure. The deposition testimony can be used to show that the witness’ trial testimony is not credible or believable.