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