CycL

CycL is an ontology language used by Douglas Lenat's Cyc artificial intelligence project, a long-running effort to build a large store of common sense knowledge. Ramanathan V. Guha was instrumental in designing its early versions, and a close variant named MELD also exists. The original version of CycL was a frame language, but the modern version is not: it is a declarative language based on classical first-order logic, extended with modal logic operators and higher-order logic quantification. CycL is the notation in which the knowledge held in the Cyc Knowledge Base, available from Cycorp, is written down. The source code written in CycL that was released with the OpenCyc system is licensed as free and open-source software, which was done to make it more useful in supporting the semantic web.