Wiktionary

Wiktionary provides definitions and language information, with entries that can include pronunciation, word origins and usage material contributed through its multilingual community.

It is useful for investigating the meaning, usage or background of a word or expression. Its focus includes Word definitions, Language reference and Community editing, with the complete entry supplying the context needed to relate a definition or explanation to the original language question.

The official Wiktionary site includes sections for Thesaurus.