Looking at the bottom of your wp-config.php file in the wordpress root directory will let you find something like this:
if ( !defined('ABSPATH') ) define('ABSPATH', dirname(__FILE__) . "https://stackoverflow.com/");
For an example file have a look here:
http://core.trac.wordpress.org/browser/trunk/wp-config-sample.php
You can make use of this constant called ABSPATH in other places of your wordpress scripts and in most cases it should point to your wordpress root directory.