The Free On-line Dictionary of Computing (30 December 2018):

expert system

    A computer program that contains a
   knowledge base and a set of algorithms or rules that infer
   new facts from knowledge and from incoming data.

   An expert system is an artificial intelligence application
   that uses a knowledge base of human expertise to aid in
   solving problems.  The degree of problem solving is based on
   the quality of the data and rules obtained from the human
   expert.  Expert systems are designed to perform at a human
   expert level.  In practice, they will perform both well below
   and well above that of an individual expert.

   The expert system derives its answers by running the knowledge
   base through an inference engine, a software program that
   interacts with the user and processes the results from the
   rules and data in the knowledge base.

   Expert systems are used in applications such as medical
   diagnosis, equipment repair, investment analysis, financial,
   estate and insurance planning, route scheduling for delivery
   vehicles, contract bidding, counseling for self-service
   customers, production control and training.

   [Difference from "knowledge-based system"?]

   (1996-05-29)