CLIPS is a public domain software tool for building expert systems. The name is an acronym for "C Language Integrated Production System." The syntax and name was inspired by Charles Forgy's OPS ("Official Production System," although there was nothing really official about it). The first versions of CLIPS were developed starting in 1985 at NASA-Johnson Space Center (as an alternative for existing system ART*Inference) until the mid 1990s when the development group's responsibilities ceased to focus on expert system technology. The original name of the project was NASA's AI Language (NAIL).
CLIPS is probably the most widely used expert system tools because it is fast, efficient and free. CLIPS incorporates a complete object-oriented language COOL for writing expert systems. CLIPS is written in C, extensions can be written in C, and CLIPS can be called from C. Its user interface closely resembles that of the programming language LISP.