Three months ago, I found some Google search shortcuts that trigger results from Google Dictionary and Google Translate. The keywords for Google Translate no longer work, but you can still add "en:en" before an English word to see the definition from Google Dictionary.
Google's results now include links to Wikipedia, Dictionary.com, Merriam-Webster and Answers.com. Two of these sites (Dictionary.com and Answers.com) have been previously used to show definitions for each word of a query. They have been replaced by Google Dictionary and the feature has been removed when Google launched a new interface, two months ago.
Here's an example of query that triggers the special definition results: [en:en overcast].
Google has another special result that's displayed if you search for [define word] or [word definition]. This time, Google shows definitions from WordNet.
There's also a define: operator that shows definitions from WordNet, Wikipedia, glossaries and other web pages.