Google's OneBox has long offered quick definitions when your search query includes text like "define flummox" or "what is flummox", but the search engine has just added "implicit triggering" that automatically offers a dictionary result when you search for single words.
It's not a big change, but it'll save you at least six keystrokes every time you want a quick definition. Not bad. For more extensive results, don't forget the always helpful define: operator.
This week in search 7/25/10 [Official Google Blog]