The multitouch feature in your Mac's laptop or your Magic Trackpad makes it easy to perform tasks that otherwise might include several taps, clicks or swipes. For example, you can look up the definition of words in documents and on webpages with a simple gesture, no need to use Spotlight or launch the Dictionary app.
A three-finger double-tap shows a word's definition and more
To check the definition of a word with a gesture, position your cursor over the word you're interested in. Now use three fingers at the same time to double-tap your trackpad. Along with the word's definition, you'll also see its thesaurus and Wikipedia entries.
This trick makes quick work of finding definitions for words you don't already know, and it's faster than typing the word in Spotlight -- which, by the way, is also a handy tip: You can type any word in the Spotlight search field to find its definition, but you'll have to click the Dictionary app in the search results.