Wiktionary
From wipipedia.org

Wiktionary is a collaborative project to produce a free, complete dictionary for the world. The goal is to define every word from all known languages in every other language, as well as in the original language itself.
Wiktionary is the lexical companion to the open-content encyclopedia Wikipedia.
The English version has about 3,700,000 entries and is continually growing. Like this encyclopedia, the content of Wiktionary is covered by the GNU Free Documentation License.
External links
- Wiktionary in Wiktionary