Professional witness

An expert witness, in England, Wales and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized (scientific, technical or other) opinion about evidence or about facts before the court within the expert's area of expertise, referred to as an "expert opinion". Expert witnesses may also deliver "expert evidence" within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts.

The earliest known use of an expert witness in English law came in 1782, when a court that was hearing litigation relating to the silting-up of Wells harbour in Norfolk accepted evidence from a leading civil engineer, John Smeaton. This decision by the court to accept Smeaton's evidence is widely cited as the root of modern rules on expert evidence. However, it was still such an unusual feature in court that in 1957 in the Old Bailey, Lord Justice Patrick Devlin could describe the case of suspected serial killer Dr John Bodkin Adams thus: "It is a most curious situation, perhaps unique in these courts, that the act of murder has to be proved by expert evidence."

Typically, experts are relied on for opinions on severity of injury, degree of sanity, cause of failure in a machine or other device, loss of earnings and associated benefits, care costs, and the like. In an intellectual property case an expert may be shown two music scores, book texts, or circuit boards and asked to ascertain their degree of similarity. In the majority of cases, the expert's personal relation to the defendant is considered and irrelevant.

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Wikipedia