For all purposes, a Game Engine is defined as a standalone tool which main purpose is to create games without the need of any assisting tool. This is by no means an universal definition, and even this definition had to be stretched a bit so we could add engines such as Fantasy Consoles. Even so, that's out general rule-of-thumb to add Game Engines on the website.

For a couple of reasons:

1. The main purpose of listing games is educational. We want Game Developers to use the existing open-source games as reference to answer questions such as "how to I organize assets in this Engine" or "how do I program using this script language".
2. It's a reliable way to validate the game. We want to make sure every game was made by the engines it refers to.
3. It gives visibility to open-source games. We want more open-source games, and we want more people knowing about them.