Longman Dictionary of Contemporary English
Wiktionary
n. (context legal English) A witness that has expertise in a certain field.
WordNet
n. a witness who has knowledge not normally possessed by the average person concerning the topic that he is to testify about [ant: lay witness]
Wikipedia
An expert witness, in England, Wales and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized (scientific, technical or other) opinion about evidence or about facts before the court within the expert's area of expertise, referred to as an "expert opinion". Expert witnesses may also deliver "expert evidence" within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts.
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/her opinion on that area.
Usage examples of "expert witness".
The morning mail held the usual junk along with a subpoena to appear as an expert witness in a custody case I'd thought long resolved, but the date was a month away, so I filed it.
I realize that Hal took great care to recruit me as an expert witness in order to make sure that I might be kept under close surveillance, but I assure you that I will be of more use to the investigation if I am allowed to follow the trail which the murderer seems to be carefully laying down for me.
I testified as an expert witness to help secure the conviction of serial murderer George Russell Jr.
That is not so unusual, I do get called in as an expert witness from time to time, as you know.
Robbie gave Evon the name of a pathologist he wanted to attend the inquest with them, an expert witness who could come to an opposite conclusion from the county coroner's, if need be.