An expert witness is a witness, who by virtue of education, profession, publication or experience, is believed to have special knowledge of his subject beyond that of the average person, sufficient that others may officially (and legally) rely upon his opinion.
A simplified example
A meteorologist present in the general area of a hurricane may be called upon as an expert witness by insurance companies or the government after the disaster subsides, to confirm the level of damage and degree of warning provided beforehand.
Experts in the real world
Typically, experts are relied on by both sides to a dispute for opinions on severity of injury, degree of insanity, cause of failure in a machine or other device, and the like.
The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties.
The expert has heavy responsibility, especially in penal trials, and perjury by an expert is a severely punished crime in most countries. The use of expert witnesses is sometimes criticized in the United States because in civil trials, they are often used by both sides to advocate differing positions, and it is left up to a jury of laymen to decide which expert witness to believe. Sometimes one side has utilized an expert witness to provide fraudulent or junk science testimony in order to convince a jury.
In most systems, the trial (or the procedure) can be suspended in order to allow the experts to study the case and produce their results.
Non-testifying experts
In the U.S., a party can hire experts to help him/her evaluate the case. For example, a car maker may hire an experienced mechanic to decide if its cars were built to specification. This kind of expert opinion will be protected from discovery. If the expert finds something that’s against its client, the opposite party will not know it. This privilege is similar to the work product protected by the attorney/client privilege.
Testifying experts
If the witness needs to testify in court, the privilege is no longer protected. The expert witness’s identity and nearly all documents used to prepare the testament will become discoverable. Usually an experienced lawyer will advise the expert not to take notes on documents because all of the notes will be available to the other party.