Read on macosxhints:
When running a Cocoa application, you can control-click on any word and select “Look up in Dictionary” from the pop-up menu. This will invoke the new Dictionary application, which will define the selected word. While this is handy, there’s an even quicker method (undocumented, as far as I can tell).
Instead of selecting the word, hover the mouse over the word and press Command-Control-D. This will pop-up a a mini-dictionary at the mouse location, containing the word’s definition, as seen at right.
Also, if you keep Command-Control pressed, the popup definition window will follow the mouse, showing a definition of any word the mouse visits.