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"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.
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Search for [define salient] and Google shows a definition from WordNet, not from Google Dictionary.
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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].