Gee, I love stuff like this, and I wish I would have known about it a long time ago. The tip comes from OSXhelp.com and it's a great one.
Need a quick definition of a word in Safari or an email? Yes, you probably already knew you could right-click on a highlighted word and bring up the OS X dictionary, but how about this? Press Command+Control+D while hovering over any word, and up pops the definition almost immediately. If you continue to hold down those keys you can slide your mouse over any other word and get a definition as well. Let go of the keys, and click somewhere else and the dictionary vanishes.
This little feature doesn't work everywhere. It requires you be in a Cocoa application, like Safari or Mail. It works in Pages, but not in MS Word because it was based on Carbon. Sadly, it doesn't work in Firefox. In fact, you can't right-click in Firefox and get a definition in the 'normal' Apple way.
If you want even more information than the little definition, click on the word 'more' at the lower right of the pop-up, and you'll get a lot more stuff from the Apple dictionary app, including usage suggestions and the origins of the word.
A couple of notes: If you are using a macro program like QuicKeys, make sure you aren't mapping the key combination you need to activate this feature, or re-map it to something else. If you click on the word 'dictionary' you can bring up the thesaurus, and if you launch the Apple dictionary application you can get into preferences and change the order of display, so you get the thesaurus as a default. You can also change your right-click behavior to open the concise panel instead of the larger definition page. Once I memorized the command key sequence I find myself using this all the time. It's quick and dirty. Let us know if you like it.
Thanks to OS X Help for all the little tips they regularly come up with, and to my fellow blogger Erica Sadun for testing this feature with QuicKeys. Sharp eyed readers will note we've covered this tip before here and here but I think it bears repeating.