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
You are the owner of a very successful car factory. You are proud of the quality of your cars. One day you find yourself in a courtroom because hundreds of your cars are found with faulty brakes. What do you do? You hire a good mechanic to tell the jury that your brakes are actually good. That mechanic is your expert witness....
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.
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