Definition - What does Expert Testimony mean?
An expert testimony is a testimony that is given by an expert who is presumed to be extremely knowledgeable about a relevant issue to the case. For example, a surgeon may give an expert testimony on a medical case where the plaintiff is suing their health insurance company. Expert witnesses are usually paid to give their testimonies.
Justipedia explains Expert Testimony
Expert testimonies are basically just when experts in fields are paid to testify in trials. The reason why experts are often called to testify in court is because they can possess knowledge that is critical to the case, but that few other people may possess. For example, a psychologist may be called to give an expert testimony in a case that involves a criminally insane person. This testimony could provide critical insight into the case.