Well this one is straight from the mother ship...
to quote Apple
So to recap whilst viewing a web page in Safari if there's a word your not sure of, just move the mouse pointer over the word and hit ctrl+command+D try it...
Learning all the time...
to quote Apple
You’re reading a pithy article on helmets.com and, suddenly, you come across a word you’ve never bumped into before. Instead of scratching your head, you could, of course, hit the trusty F12 key and look up the meaning of the word in the Dictionary Widget.
But here’s a cool alternative. Hold down, all at once, the Control, Command, and D keys while pointing your mouse directly at the word in question. And, voilà, up pops a little window with a definition straight out of the new dictionary built into Mac OS X Tiger...
So to recap whilst viewing a web page in Safari if there's a word your not sure of, just move the mouse pointer over the word and hit ctrl+command+D try it...
Learning all the time...