The following constants are defined by ExpressionEngine as of 1.6.8:
* APP_BUILD
* APP_NAM
* APP_VER
* EXT
* LD
* PATH
* PATH_CACHE
* PATH_CORE
* PATH_CP
* PATH_CP_IMG
* PATH_DB
* PATH_DICT
* PATH_EXT
* PATH_LANG
* PATH_LIB
* PATH_MBR_THEMES
* PATH_MOD
* PATH_PI
* PATH_SITE_THEMES
* PATH_THEMES
* PATH_TMPL
* RD
* REQ
* SELF
* SLASH
* USER_BLOG
To get a complete list of system-wide contants, run
<?php
print_r( get_defined_constants(TRUE) );
?>