CodeIgniter 2.0 (Reactor) Error on ENVIRONMENT Constant

Environment: 
CodeIgniter PHP Framework (obviously, on a PHP-enabled Web Server)
Problem: 

Upgrading CodeIgniter from "early 2.0" to a release after the Core/Reactor split resulted in a "use of undefined constant ENVIRONMENT" error.

Status: 
Fixed
The Fix: 

Make sure the new index.php file is used (edit to use custom settings as needed).

The human error that led to the error message was replacing the "system" directory but forgetting to update the index.php file, which now defines an ENVIRONMENT constant for differentiating between development, testing, and production.