Using Mac's built-in dictionary
Did you know that Mac has a very handy built-in dictionary?
Press and hold Apple+Control+D over any word in a cocoa application and up pops a Dictionary/Thesaurus. You can even move your mouse over any other word on the page and it will keep throwing out definitions. Sorry to say that this trick only works on cocoa applications in Mac OS X such as the Address Book, Calculator, Chess, iCal, iChat, iTunes, Mail, Preview, QuickTime, Safari, Stickies and TextEdit.