What is CWD in require_once(CWD . '/includes/class_core.php');

[doob]

Hacking external login today! And as always curious about some of the obscure code I'm finding.

Anyone know what CWD stands for or does in this line in init.php?
require_once(CWD . '/includes/class_core.php');

I imagine its functioning as a definition of a root directory, but its not readily obvious.

Also curious about lack of variable on DIR as in require_once(DIR . '/includes/class_database_slave.php');

Again, I imagine that DIR is definted somewhere as homeDirectory or Root or similar, but I'm not familiar with this being done in PHP without a $.