An expert system is piece of software designed to embody some elements of expert knowledge and reasoning in order to aid or act as substiture for an expert. For example, an expert system might embody diagnostic knowledge of rare diseases that can be used to advise a doctor.
Used in glossary entries: expert knowledge, reasoning

Different forms of expert involvement.

Expert system capturing and applying knowledge

Hybrid expert system incorporating machine learning guided by human expertise.

Typical expert system architecture