Cyc is an artificial intelligence project that attempts to assemble a comprehensive ontology and knowledge base of everyday common sense knowledge, with the goal of enabling AI applications to perform human-like reasoning.
The project was started in 1984 by Douglas Lenat at MCC and is developed by the Cycorp company. Parts of the project are released as OpenCyc, which provides an API, RDF endpoint, and data dump under an open source license.
Read more about Cyc: Overview, Knowledge Base, Inference Engine, Criticisms of The Cyc Project