The Quake engine is the game engine developed by id Software to power their 1996 video game Quake. It featured true 3D real-time rendering. Since 2012, it has been licensed under the terms of GNU General Public License v2.0 or later.
Ingame screenshot of the first-person shooter Nexuiz, running on a modified Quake engine
A game engine is a software framework primarily designed for the development of video games and generally includes relevant libraries and support programs such as a level editor. The "engine" terminology is akin to the term "software engine" used more widely in the software industry.
Creating a platform game in Godot