Unity is a cross-platform game engine with a built-in IDE developed by Unity Technologies.
It is a commercial development platform for creating apps, games and simulations with 3D, 2D, VR and AR content.
- Stackoverflow.com Wiki
A 2D/3D engine and framework that gives you a system for designing game or app scenes for 2D, 2.5D and 3D. Unity allows you to interact with them via not only code, but also visual components, and export them to every major mobile platform and a whole lot more - for free.
When creating a game that contains levels, the last thing a developer would like to do is to code every single level. Instead what he wishes to do is to create independent game objects which he can put together in different combinations and levels using easily configurable XML files.
Unity is the award-winning development platform for games and interactive content on desktop, iOS, Android, consoles and beyond.