Dictionary Bubble is a free Chrome extension that provides definitions and synonyms for any word that you double-click in an online article. The extension will also provide an audio pronunciation of the words on which you double-click. If highlighting and or using keyboard commands is more your style, you can customize the extension to response to highlights and keyboard shortcuts.

Applications for Education
Dictionary Bubble could be useful to students who need a little assistance with the definitions and or pronunciations of new words that they encounter in an online reading assignment.

H/T to Addictive Tips.