OpenCyc is the open source version of the Cyc technology, that is an ontology containing hundreds of thousands of Cyc terms covering all human consensus reality domains. It is available for download under an OpenCyc License. Release 4.0 of OpenCyc includes:

• ~239,000 terms
• ~2,093,000 triples
• Select class (direct and indirect) instance counts
• ~69,000 owl:sameAs links to external (non-Cyc) semantic data namespaces
• English strings (a canonical one and alternatives) corresponding to each concept term, to assist with search and display.
• A java-based Cyc Inference Engine and the Cyc Knowledge Base Browser.
• Documentation and self-paced learning materials to help users achieve a basic- to intermediate-level understanding of the issues of knowledge representation and application development using Cyc
• A specification of CycL, the language in which Cyc (and hence OpenCyc) is written. There are CycL-to-Lisp, CycL-to-C, etc. translators.
• A specification of the Cyc API, by calling which a programmer can build an OpenCyc application with very little familiarity with CycL or with the OpenCyc KB.