So as Jeff Gamet mentioned last year, you can use your trackpad to tap a word with three fingers (or three-finger double-tap under Lion) to get a quick definition.
Here I’ve opened the pop-up, but I can’t see the entire thing because it’s been cut off. What to do? Well, I can click on the definition text itself, and the window will expand to show more details.
But as the aforementioned awesome reader noted, the headers within the pop-up are actually clickable, and when you do that, you’ll bop right into the Dictionary program to view the entire article.
Nifty! Now, if the three-finger tap (or double-tap) thing isn’t working for you at all, make sure it’s enabled under System Preferences> Trackpad> Point & Click.
Also, here’s another tip on this feature. If you wanna enable different dictionaries (or stop Wikipedia from appearing on the pop-up at all, say), you do that within the Dictionary program’s Preferences.
Once you toggle Wikipedia off as I've done, the three-finger pop-up won’t show it at all. Heck, if you want to see nothing but the definition, turn everything else off within that same Preferences window. That'll lead to some clean pop-ups, let me tell you.
Feeling especially crazy? Then you can also drag the items in that Preferences list into a different order, so that the three-finger pop-up shows a thesaurus first, for example.
On an unrelated note, doesn't “three-finger pop-up” sound like the name of a rude gesture you'd make at someone you don't like? It totally does. I promise I'll never make the three-finger pop-up at you, dear readers.