Prime purpose of this article is to Define and Discuss on Expert System. An expert system has a distinctive structure, different from traditional computer-programming. It is divided straight into two parts, one fixed, independent of the professional system: the inference engine, and one variable: the knowledge base. In artificial intelligence, an expert system is a computer system that emulates the decision-making ability of a human expert. Expert systems are designed to solve complex problems by reasoning about knowledge, as an expert, and not by following the procedure of a developer as could be the case in conventional development.