
For now, this feature only works for language pairs that include English: en:fr, fr:en (French), en:it, it:en (Italian), en:de, de:en (German), en:es, es:en (Spanish), en:ru, ru:en (Russian), en:zh, zh:en (Chinese), en:ja, ja:en (Japanese), en:ko, ko:en (Korean).
If you use the language pair en:en (English to English), Google shows definitions from Google Dictionary. For example, a search for [en:en astute] shows the definition of the word "astute" and the pronunciation. Google Dictionary uses definitions from Collins COBUILD Advanced Learner's English Dictionary.

You can also search for [astute definition], but the OneBox result shows definitions from WordNet.