Expert witnesses, also known as professional witnesses, are witnesses called in a court situation to give their opinion on a matter. Expert witnesses may be from any walk of life, and may have any area of expertise, but they must be widely accepted as experts in their field. Being expert means having knowledge and understanding about their field well in excess of that possessed by an ordinary person, to such a degree that the court can give special weight to their opinion and treat it nearly as fact.
The use of expert witnesses is crucial in many cases, because often there are not clear and objective facts to rely on. In these cases, a subjective opinion must be sought in order to make a determination of a situation. Given that, the court must rely on someone to give their opinion, and that person must have sufficient credibility in the world at large to make it likely they will not lie or be incorrect in their assessment of a situation. In some cases expert witnesses may deliver their opinion, only to have it later rebutted by what is called a learned treatise, in which case their classification as expert may be called into question.
Of course, within the domain of an expert witness there is often significant room for subjectivity, so in many cases multiple expert witnesses will be brought in. Often two sides of a case will bring in their own expert witnesses, who may disagree completely with one another. In this case, more experts may be brought in, or a determination may be attempted as to which expert holds the most credibility.
The opinion of an expert witness, referred to officially as an expert opinion, may be used in all sorts of cases. One of the most well-known uses in the modern world is that of psychiatrists or psychologists giving their expert opinion on whether or not a defendant is mentally insane, and to what degree. Another widely-known use is to have a doctor give their opinion on how serious an injury is. Experts in more obscure fields may also find their opinion valued by a court, particularly in intellectual property cases. For example, an expert on music might be played two songs to give their opinion on how similar they are within the scope of the genre.
Both sides in a case may use expert witnesses, and in many cases they may use experts without actually having them appear to testify. These non-testifying experts can be used to discern the strength of an argument one side may be debating whether or not to employ. Their expert opinions are protected from discovery, so that if their opinion turns out to work against the person who hired them, they do not have to disclose it to the other side. Once the expert is decided on to testify, of course, this protection disappears, and both the witness and the work they did to form their opinion becomes discoverable to the other side.