Garth Kemerling formerly of Newberry College has skillfully crafted a comprehensive and thoroughly cross-linked dictionary of philosophical terms and persons. For students of philosophy, this site should be a first reference.

Dictionary of Philosophy

Edited by Dagobert D. Runes, the dictionary includes terms from ancient, medieval, and modern philosophy with terms from Eastern philosophy as well — with definitions by over 70 notable philosophical specialists. Although dated in some respects, this work is one of the best on the web or short accurate entries.

The Internet Encyclopedia of Philosophy

This site, by James Fieser and Bradley Dowden, is easily accessible by an index containing short articles on some philosophical concepts and many philosophical personages drawn from the public domain, from the editors themselves, or from original submissions.

Edited by Edward N. Zalta of the Metaphysics Reseach Lab has entries maintained by authorities in the field. Although still very much under construction, the emphasis of their entry-selection is on the concepts of philosophy