Google added a OneBox result for definitions to all global English users, that using data from Google Dictionary, shows "pronunciation information, short definitions and links to other reference sites like Dictionary.com and Answers.com." Google shows definition of an English word only if it's likely to be useful, so you'll usually see 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. Stay tuned for more enhancements here, including an expanded mobile experience," explains Google.
To trigger new OneBox when it's not displayed, add en:en to your query.