I've been using this trick for a while now and thought I might share it with all of you. You can quickly look up the definition of a word on your Mac by tapping on it with three fingers. If you're using Mac OS 10.8 Mountain lion then simply place your cursor on top of a word then tap on your trackpad with three fingers at once. Your mac will look up that word in the Oxford American dictionary and thesaurus and show you the results in a pop-up window. If you are using Mac OS 10.7 Lion then you will need to tap twice instead of once.
I've included this very short video to show you how it works.