Mac OS X has a great keyboard shortcut to access the system dictionary for a quick definition of a word. Press the keys Command-Control-D when the cursor is hover over the word, and a pop-up window will display the definition and thesaurus for the word.
Mac OS X Lion expands on this convenience by giving us a gesture shortcut for Mac notebooks or Mac with a trackpad. Simply double tap with three fingers on the trackpad when the cursor is over the word. The word will be highlighted, and a pop-up definition box will appear. In Mac OS X Lion, the pop-up is enhanced with easy access to the dictionary and thesaurus of the word. And a new Wikipedia entry for the word is added.
Dictionary pop-up is a system wide feature and will work in most Lion native apps such as Safari and TextEdit. This feature will also work in third party apps that make use of Cocoa text components.
Note: OS X Mountain Lion changes the behaviour to single tap with three fingers instead of double-tap.