Mac OS X has a built-in dictionary, just like iOS. You can highlight any word anywhere and look its definition up by right-clicking it and selecting 'Look up "word"'. However, there is also a gesture shortcut in Mac OS X Lion and above that lets you do this a bit more efficiently.
When you want to instantly look up the definition of a word in any piece of text, mouse over it and use three fingers to double-tap on the word. The dictionary definition should pop right up.
Obviously, this tip works on a Mac with a touchpad connected.
Use Three Finger Taps to Look up Words in the Dictionary Quickly on Mac OS X
