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.
Note the need to use three slashes, not two as most protocols require. Also, if you misspell the word, the Dictionary application will open (or come to the front if it is already open) but will not find anything. (This differs from when you misspell a word directly within Dictionary -- when you do that, you'll usually see "No entries found. Did you mean?" followed by a list of possible matches.)

