Virtually it is not one lexicon but a whole collection of various kind of dictionaries and lexicons. Some of them for instance:
- Webster's Revised Unabridged Dictionary
- WordNet English Lexical Database
- FOLDOC Dictionary of Computing
- VERA Acronym Dictionary
- U.S. Gazetteer Places and Zip codes
and more.
jDictionary's Lexicon Online Plugin offers you a powerful, easy to use, platform-independent and free interface for these famous dictionaries.

It contains almost 80MB of pure data. Of course you don't need to download this whole lot of data. The data stays on the DICT server and your Lexicon Plugin will query that server for information.

Lexicon Online is as easy to use as a modern internet browser. Contains navigation, links between words, advanced caching technology, tight integration with jDictionary, proxy support and more.

Of course, jDictionary can install the Lexicon Plugin automagically from the Net. Just one click in the Plugin Manager and in a few minutes the Lexicon Plugin is ready to use.
Plugins menu -> Download & Install Plugins -> Click the Download & Install button at the Lexicon list item.
Be sure your computer is online. That's all there is to it.