Expert Witness

n. professional person who gives relevant testimony from the field of his expertise (Law)

An expert witness is a witness, who by virtue of education, 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.
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Noun
1. a witness who has knowledge not normally possessed by the average person concerning the topic that he is to testify about
(antonym) lay witness
(hypernym) witness

A specialist who, on the basis of having special knowledge in a particular field, testifies in a legal proceeding.

Person employed to give evidence on a subject in which they are qualified or have expertise