An
expert witness,
professional witness or
judicial expert is a
witness, who by virtue of
education,
training,
skill, or
experience, is believed to have
expertise and specialized
knowledge in a particular subject beyond that of the average person, sufficient that others may officially and legally rely upon the witness's specialized (scientific, technical or other) opinion about an
evidence or
fact issue within the scope of his expertise, referred to as the expert opinion, as an assistance to the
fact-finder. Expert witnesses may also deliver
expert evidence about facts from the domain of their expertise. At times, their testimony may be rebutted with a
learned treatise, sometimes to the detriment of their reputations.