Automatically look up Apple’s Dictionary in Cocoa Applications
Mac OS X comes with a built in dictionary, which can be launched from your Applications Folder whenever you need to look a word up. However, there is another way to accomplish this without even launching Apple Dictionary. Pressing and holding ‘Apple + Control + D’ in any Cocoa application will result in a Dictionary/Thesaurus popping up. Move your mouse to another word while holding down the keys to search for new definitions.
If pressing down the keys and hovering your pointer over a word does not work for you, verify that the “Look up in Dictionary’ item is checked in ‘System Preferences’ > ‘Keyboard & Mouse’ > ‘Keyboard Shortcuts’.