Expert Witness - a person who is a specialist in a subject, often technical, who may present his/her expert opinion without having been a witness to any occurrence relating to the lawsuit or criminal case.
It is an exception to the rule against giving an opinion in trial, provided that the expert is qualified by evidence of his/her expertise, training and special knowledge. If the expertise is challenged, the attorney for the party calling the "expert" must make a showing of the necessary background through questions in court, and the trial judge has discretion to qualify the witness or rule he/she is not an expert, or is an expert on limited subjects.
Experts are usually paid handsomely for their services and may be asked by the opposition the amount they are receiving for their work on the case. In most jurisdictions, both sides must exchange the names and addresses of proposed experts to allow pre-trial depositions.
Pittsburgh Criminal Defense Attorney Frank Walker