Google added a new OneBox result for definitions. The OneBox uses data from Google Dictionary, shows pronunciation information, short definitions and links to other reference sites like Dictionary.com and Answers.com. Google shows the definition of an English word only if it's likely to be useful, so you'll usually see the OneBox when you search for obscure words or technical terms.
"We 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]. We've also improved the definition result snippet to show more details such as parts of speech and pronunciation," explains Google.
Unfortunately, Google's new OneBox is redundant and inconsistent. If you type [salient] in Google's search box, Google Suggest already shows a definition of the word from Princeton's WordNet.
Search for [define salient] and Google shows a definition from WordNet, not from Google Dictionary.
Tip: to trigger the new OneBox when it's not displayed by default, add en:en to your query. For example, search for [en:en emulsion].