This is a simple hint, but one that can be very useful. You can look up any word is the OS X Dictionary application by typing the following into the address bar of your favorite browser:
dict:///word_to_define
For example, dict:///sesquipedalian will display the meaning of the word sesquipedalian in Dictionary, if Dictionary is already running. If not, it will simply open Dictionary. So to look up words, you need to first open the application, then the protocol will be available.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060907051658239