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 specialised 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.
In Scots Law, Davie v Magistrates of Edinburgh (1953) provides authority that where a witness has particular knowledge or skills in an area being examined by the court, and has been called to court in order to elaborate on that area for the benefit of the court, that witness may give evidence of his opinion on that area.