An expert witness is a witness, who by virtue of eucation, training, skill, or experience, is believed to have 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 their 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.