Witness impeachment, in the law

Law

Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

 of evidence

Evidence (law)

The law of evidence encompasses the rules and legal principles that govern the proof of facts in a legal proceeding. These rules determine what evidence can be considered by the trier of fact in reaching its decision and, sometimes, the weight that may be given to that evidence...

, is the process of calling into question the credibility of an individual who is testifying in a trial

Trial (law)

In law, a trial is when parties to a dispute come together to present information in a tribunal, a formal setting with the authority to adjudicate claims or disputes. One form of tribunal is a court...

. There are several ways that a witness may properly be impeached and several other ways that, although effective, are prohibited except under special circumstances.
A party may be impeached through introducing evidence of any of the following (remembered via the mnemonic BICCC):

• Bias--The witness is biased against one party or in favor of the other.