A witness who has special knowledge of the subject about which he is to testify. That knowledge must generally be such as is not normally possessed by the average person. An expert witness may be called on to testify in certain types of cases, such as calling upon a medical expert for medical malpractice cases or a vocational expert for social security disability cases.
This expertise may derive from study and education, or from experience and observation. An expert witness must be qualified by the court to testify as such. To qualify, he need not have formal training, but the court must be satisfied that the testimony presented is of kind which in fact requires special knowledge, skill or experience. Such testimony, given by an expert witness, constitutes expert evidence or expert testimony. Hypothetical questions (asking the witness to assume certain stated facts) may be asked of an expert witness as a way of educating the Trier of fact in the area of the expert’s knowledge or experience.