I’m not sure whether this is something new, but apparently Google has an online dictionary that lets you find definitions and synonyms in almost thirty different languages, pulled from a scope of different sources and impressively fast.
Previously, Google used to offer up an automatic definition from various third-party sources like Dictionary.com or Answers.com. However, now as you can imagine there’s a little blue "definition" link on the right side of any word or phrase search, which links directly to Google’s own homebrew definition answers.
Google does have a special keyword called “Define:keyword” that lets you search for the definition of any word almost instantaneously. The “define” operator is still pulling data from various 3rd party sources instead of Google’s own dictionary. But I’m sure this is going to change in the near future.
Check out the Google Dictionary: http://google.com/dictionary
If you have any feedback about this new tool, do send them via comments!