Refer to the tip published last year on Double tap a word with three fingers to get pop-up dictionary, thesaurus and Wikipedia in Mac OS X Lion. In OS X Mountain Lion, instead of double-tap, Apple changed this behaviour to use single tap with three fingers.
Single tap with three fingers on the trackpad when the cursor is over a word. The word will be highlighted, and a pop-up definition box will appear with dictionary, thesaurus and Wikipedia of the word. This definition pop-up is a system wide feature and will work in most apps. OS X Mountain Lion uses the same keyboard shortcut Command-Control-D for the definition pop-up.