Metaglossary.com "harvests definitions from the Web". This is as good or better as Google's define: command and certainly better than searching for the meaning of a term through a keyword search at a search engine. MetaGlossary also has a small box for looking up non-English words through Babylon.
From the about page:
"MetaGlossary is able to precisely extract the meanings of terms and phrases from the often frustratingly unmanageable mass of information on the web. ...
What's more, MetaGlossary organizes these meanings based on topic and usage, so you'll find the one you're looking for quickly and easily. "