Expert testimony refers to evidence presented by an expert witness in a legal proceeding. An expert witness is typically someone who has specialized knowledge or experience in a particular field, and they are called upon to provide their opinion on matters related to that field. The purpose of expert testimony is to help the trier of fact (jury or judge) understand complex issues by providing an explanation based on their area of expertise. Expert witnesses may be used in both civil and criminal cases, depending on the nature of the case and the relevance of their expertise.