OPS5 is a rule-based programming language used for artificial intelligence and expert systems. It was created in the mid-1970s and popularized by the LISP community as a way to develop production systems. OPS5 uses pattern matching to trigger specific responses or actions based on predefined rules. This allows developers to model the decision-making process of human experts in fields such as medicine, finance, and engineering. Although not widely used today, OPS5 laid the foundation for other rule-based AI systems by illustrating how knowledge can be represented as rules within an inference engine.