Software that aims to emulate the decision-making capacities of human experts.
There was a boom in rule-based expert systems in the 1980s, many of them based on logic programming. At the time they were a big part of what people meant by AI. Late 80s often regarded as a period of collapse for expert systems, "AI winter". That label sometimes disputed on the grounds that the techniques of expert systems continued to develop and see use, but under other names.