MAC: Command+Control+D has always been a handy shortcut for looking up the definition of a word on the Mac, but now in Yosemite you can look up a heck of a lot more, including suggestions from iTunes and the App Store.
All you need to do is highlight a word in OS X, then tap Command+Control+D (or right-click and choose "Look Up in Dictionary"). If you happen to select a word that fits into one of the contextual searches, you'll get specific results for it, much like you would if you searched in Spotlight. Handy!
OS X Yosemite (10.10.3) Has a New Hidden Spotlight Feature [Jacob Salmela]