A technique used during cross-examination to discredit the testimony of a witness.  Impeachment can be accomplished in a number of ways:  by demonstrating and emphasizing the difference between the witness’s testimony at trial and a prior statement, by showing bias, or by showing erroneous assumptions made by the witness in drawing conclusions, etc.  The intent of impeachment is to show the jury that the witness cannot be believed such that their testimony will be disregarded or given little weight.