Tip #7 : WordPress constants
Posted by Leonid Mamchenkov on October 5, 2007
ABSPATH is one of the most known WordPress constants. It holds filesystem path to WordPress installation and is often used to construct filesystem paths relative to WordPress. But what other constants are there in WordPress? The answer is easy to get than it might seem. You can see a full list here.
(Thanks to Benedict Eastaugh over at wp-hackers mailing list for the above link)