Expert witness in the law of evidence, a witness who is allowed to give opinion evidence as opposed to evidence of his perception. This is the case only if the witness is indeed skilled in some appropriate discipline. An exception to the usual rule of practice whereby witnesses are heard one after the other and do not hear the evidence of the preceding witness is made in relation to competing experts.
While lay witnesses tend to deliver evidence like dates, time-lines, visual identification and general information, expert witnesses step it up a notch. They bring their expertise to the table, providing case-relevant opinions which have the potential to make or break a case.
An expert witness is a witness who has knowledge beyond that of the ordinary lay person enabling him/her to give testimony regarding an issue that requires expertise to understand. Experts are allowed to give opinion testimony which a non-expert witness may be prohibited from testifying to. In court, the party offering the expert must lay a foundation for the expert's testimony. Laying the foundation involves testifying about the expert's credentials and experience that qualifies him/her as an expert. Sometimes the opposing party will stipulate (agree to) to the expert's qualifications in the interests of judicial economy.
Experts are qualified according to a number of factors, including but not limited to, the number of years they have practiced in their respective field, work experience related to the case, published works, certifications, licensing, training, education, awards, and peer recognition. They may be called as upon as consultants to a case and also used to give testimony at trial. Once listed as a witness for trial, the materials they rely upon in forming an opinion in the case is subject to discovery by the opposing parties. Expert testimony is subject to attack on cross-examination in the form of questioning designed to bring out any limitations in the witness's qualifications and experience, lack of witness's confidence in his opinions, lack of the preparation done, or unreliability of the expert's sources, tests, and methods, among other issues.