As you can see, it is placed at the top of the results and has links to four online dictionaries. The dictionaries include Dictionary.com, Answers.com, Merriam-Webster & The Free Dictionary.
Google wrote about the change on Sunday. They didn't mention the new look of the result or the new links added, but did say they added "implicit triggering, which means you can simply search for [flummox] and find the definition, you don't have to search for [define flummox] or [what is flummox]." Google also said they "improved the definition result snippet to show more details such as parts of speech and pronunciation."
Forum discussion at WebmasterWorld.