From Longman Dictionary of Contemporary Englishexpert witnessˌexpert ˈwitness noun [countable] someone with special knowledge about a subject who is asked to give their opinion about something relating to that subject in a court of law He appeared as an expert witness before several government inquiries.
Examples from the Corpus
expert witness• He appeared as an expert witness before several government inquiries, and was a commissioner on that investigating mine safety in 1882.• Ask a pathologist, find an expert witness.• The Court agreed with the Special Commissioner over the admissibility of the evidence of the taxpayer's expert witness.• First, the expert witness must be paid for his time.From Longman Business Dictionaryexpert witnessˌexpert ˈwitnessLAW someone who has expert knowledge in a particular area and comes to a court of law to give their opinion about itHe appeared as an expert witness before several government inquiries. → witness 