Under U.S. law, an expert witness is someone whose opinion on a specific matter before a court of law is considered “expert” by the court because of that individual’s education, scholarship, and/or overall experience. Any testimony or evidence given by the expert witness is called an “expert opinion,” is submitted to the case as evidence, and can be rebutted in court by witnesses, conflicting evidence, or other experts.
Different countries have different standards for who can qualify as an expert witness and what the person can testify about. In the United States, each state has its own laws or guidelines about expert witness testimony in its state and local courts.
Additionally, while some expert witnesses may testify as an “objective” witness for the court’s and the jury’s benefit, individual parties in a court case can hire or consult with their own experts to prepare reports for the court or to testify, to provide information or argument on a client’s behalf.