It is possible to access the Mac OS X Dictionary and Thesaurus from any Cocoa application (Mail, TextEdit, Safari, etc.) using a simple keyboard shortcut.

To access the dictionary, place the mouse cursor over the word you want to look up., and press Command-Control-D. a small window pops up giving you a quick definition, and you can easily select thesaurus or Morefor more information on a word.

To look up multiple words you can just move the mouse whilst keeping command and control held down (you can release D).