We all know that the law offers no shortage of difficult words, and we keep our law dictionaries handy for the occasions when we encounter them. For quick, online definitions of non-legal difficult words (although a few legal terms of art may appear here, as well), however, try the Online Dictionary of Difficult Words. You may search for a particular word by typing it into the search box, or you may browse the list of words alphabetically. From a capella to zymotic, many difficult, unusual words and their definitions appear here, including my own favorite, ailurophile.
Ernster, the Virtual Library Cat