A witness is a person who testifies under oath at a trial, or in a deposition, regarding experiences of which he or she has personal knowledge. A witness gives a supervised recital of things he or she experienced, whether by sight, hearing, smell, or other sensory perception. A witness may voluntarily offer such information in a legal matter, or may be compelled to testify. Witnesses may testify in both civil and criminal legal matters. To explore this concept, consider the following witness definition.

Definition of Witness


  1. An individual who is present, and personally perceives or sees something
  2. A person who testifies in a court of law
  3. A person that presents or affords evidence


  1. To personally see, hear, or know something
  2. To be present at an event as a spectator or bystander
  3. To testify or provide evidence


Pre-950 A.D.    Old English witnes  (knowledge, understanding)

What is a Witness

In legal matters, a witness is an individual who has knowledge relevant to the case, but may also be someone who claims to have, or who is thought by others to have, such knowledge. A witness may be sworn under oath to testify to his knowledge and details of the case, including things he has seen, heard, smelled, or touched first hand.

A person who testifies about what someone else told him, or what someone else said, wrote, or did, is referred to as a “hearsay” witness. Hearsay testimony is very limited as to what may be presented in court.

For example:

John gets into a fight while playing pool at the corner bar. During the fight, someone was stabbed, and John has now been charged with the crime. During trial, both the prosecution and defense are likely to call people who were present at the bar when the stabbing took place to testify about the incident. The jury will rely, in part, on testimony of these witnesses in making their decision.

Witness for the Prosecution

A witness for the prosecution is used in state or federal criminal court cases. These witnesses are called to provide testimony in support of the prosecution’s case against the accused. Law enforcement officers and various experts are often called to serve as witness for the prosecution.

When a criminal trial takes place, the prosecution must convince the judge or jury that the defendant is guilty of the crime. While presenting the case, the prosecutor submits evidence gathered during the investigation, and calls witnesses, who testify about what they say, heard, or otherwise know about the case. Many times, the victim in a case serves as witness for the prosecution, as the information they provide is deemed credible and detailed.

An individual who serves as witness for the prosecution may be questioned by the defendant (or his attorney), in what is known as “cross-examination.” This gives the defense an opportunity to establish the fact that the witness gave false, biased, or inconsistent testimony. Giving false testimony is known as “perjury,” and can result in criminal charges against the witness.

Expert Witness

An expert witness is an individual who has specialized knowledge or skills relevant to the matter at hand. Expert witnesses are used to make sense of complex evidence, such as scientific data, or to explain complicated matters to the judge or jury. For example, an expert witness may be called to explain how fingerprint-matching techniques are used, and how the prosecution determined the fingerprints of the defendant match those left at the crime scene.

An expert witness may be a doctor, scientist, specialist, or other professional. Unlike regular witnesses, expert witnesses are allowed to give professional opinions during testimony. When cross-examined by the opposing party, an expert witness may be questioned on his knowledge and skills, the number of years he has been in his field, and what experience he has that makes him an expert on the matter.

Educating Witness

An educating witness teaches the jury or judge about a scientific theory that pertains to the case. This type of witness is only called to give an opinion on the validity of a theory, and on the reliability of scientific instruments and tests. An educating witness must be accredited as an expert witness in his specific field, which requires academic qualifications or specialized training.

Reporting Witness

A reporting witness is called to testify after the educating witness has completed his testimony. Reporting witnesses are often technicians or other individuals who actually conducted certain tests on evidence, such as DNA, or other scientific evidence. The reporting witness describes the actual test, and the results obtained.

Hostile Witness

A hostile witness, sometimes called an “adverse witness,” is an individual who, after being called to the stand and sworn in, appears unwilling or reluctant to tell the truth. In most cases, when either side calls a witness, it is done with the expectation that the witness’s testimony will be in line with statements he made prior to the trial. If a witness instead lies on the stand, or refuses to answer questions, the attorney can ask the judge to declare him a hostile witness. If the judge grants the request, the attorney is then allowed to ask leading questions, which are otherwise only allowed during a cross-examination, not questioning of the party’s own witness.

For example:

During the investigation phase of an arson case, Daniel tells the prosecutor that he saw Bob running away from the building with a gas can in his hand right before flames were seen in the building. When called to testify during the trial, Daniel appears reluctant to say he saw Bob, saying instead that he doesn’t remember. The prosecutor may ask that Daniel be declared a hostile witness, which will give him the ability to ask leading questions, in an attempt to get Daniel to provide helpful testimony.

Silent Witness

The silent witness theory pertains to the introduction of photographs or video recordings as evidence, without first having a witness verify its authenticity. The silent witness theory holds that, when a solid basis is provided to assure the accuracy of a photograph, it can be used as testimony, to speak for itself. This legal theory, silent witness, is based on the ability to provide solid proof of the validity of the photograph or recording, negating the need to have a human witness testify.

Material Witness

The term material as it relates to the law, refers to a fact that is significant to the case, whether in a civil lawsuit, or a criminal matter. The testimony of a material witness is someone who has knowledge of something that is very important to determining the outcome of the case, and because of this, the court must make every effort to allow the witness to give his testimony.

Efforts to ensure testimony of a material witness may include continuing the trial, which is postponing the proceedings temporarily, if the witness is temporarily unavailable. In addition, if the material witness is likely to flee in an effort to escape testifying, the court may issue a warrant to have the individual detained until he has testified.

Witness Protection

Witness protection is a service offered in order to protect threatened witnesses, or any other person involved in a court case. Witness protection may be provided to protect a witness throughout the trial process, and even after the trial has been completed in some cases. Witness protection is usually provided by local police or other law enforcement officials. While witness protection typically concludes after a trial has ended, there are cases in which a material witness needs to be provided with a new identity, and given a new life in which he continues to live under government protection.

For example:

Nick witnessed members of a local motorcycle gang threaten, then beat the owner of a bakery in his neighborhood. The prosecutor wants Nick to testify in court about what he saw, but Nick is afraid the gang will harm him if he does. Nick may be placed in witness protection, guarded by police officers, to keep him safe until the trial has ended, in exchange for his agreement to testify.

The United States Federal Witness Program is a program administered by the Department of Justice, and operated by the U.S. Marshals Service, to provide protection to witnesses. While some states have instituted their own witness protection programs, it is commonly the U.S. Marshals Service that provides ongoing protection, including new identities. States operating witness protection programs include California, Illinois, New York, and Texas.

Battle of the Expert Witnesses in Daubert v. Merrell Dow Pharmaceuticals, Inc.

Jason Daubert and Eric Schuller were both born with limb-shortening birth defects, which their parents claimed were linked to the mother’s ingestion of an anti-nausea drug, Bendectin, which was prescribed to her during her pregnancy. In 1983, after hundreds of lawsuits were filed against the pharmaceutical company that made the drug, Merrell Dow Pharmaceuticals, Inc., pulled it off the market.

Daubert and Schuller sued Merrell Dow Pharmaceuticals in a California State Court. The company introduced an expert witness who testified that there was no published study that demonstrated a link between the medication and limb-shortening, or “teratogenic,” birth defects. Daubert and Schuller introduced eight of their own expert witnesses, all of whom testified that their own unpublished studies demonstrated a link between Bendectin and this type of birth defect. Additionally, two of these expert witnesses testified that re-analyzing the original tests could support the link between the drug and teratogenic effects.

The California District Court ruled in favor of Merrell Dow Pharmaceuticals request for Summary Judgment. Daubert and Schuller appealed. The appellate court affirmed the lower court’s decision, stating that, because the methodologies used by the plaintiff’s expert witnesses had not been published, they were not generally accepted within the scientific community as reliable techniques. This is one standard by which courts could decide on whether an expert witness’s testimony is reliable.

The U.S. Supreme Court agreed to review the case on the belief that there was no uniformity concerning expert testimony in the lower courts. The Court ruled that the Federal Rules of Evidence should determine the admissibility of expert testimony, not the standard of “generally accepted by the scientific community.” The Court ruled that any testimony by an expert witness, which is relevant in assisting the jury with its decision, should be deemed reliable. The court offer suggestions on what the lower court should consider when determining the reliability of expert testimony. These considerations include determining whether the theories and techniques employed by the expert:

  • have been tested
  • been subjected to peer review and published
  • have a known and documented error rate
  • are subject to standards governing their use
  • are widely accepted within the scientific community

Based on this, the Supreme Court remanded the case back to the lower court, with the instruction to utilize the new standards in making its decision.

Related Legal Terms and Issues

  • Civil Lawsuit – A lawsuit brought about in court when one person claims to have suffered a loss due to the actions of another person.
  • Criminal Proceedings – A legal process to prosecute an individual charged with the commission of a crime.
  • Defendant – A party against whom a lawsuit has been filed in civil court, or who has been accused of, or charged with, a crime or offense.
  • Leading Question – A question that prompts or encourages a witness to give a desired answer.
  • Opinion – A judgment formed about something which is not necessarily based on knowledge or fact.
  • Plaintiff – A person who brings a legal action against another person or entity, such as in a civil lawsuit, or criminal proceedings.
  • Trial – A formal presentation of evidence before a judge and jury for the purpose of determining guilt or innocence in a criminal case, or to make a determination in a civil matter.
  • Victim – A person who is injured, killed, or otherwise harmed as a result of a criminal act, accident, or other event.
  • Warrant – A writ issued by a court or other legal official authorizing law enforcement to make an arrest, search a premises, or take some other action related to the administration of justice.

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