Also known as: Unity, ユニティ
A 2D & 3D game engine made by Unity Technologies.
It's designed for extensive scalability with support for all major platforms, meaning it's possible (within a reasonable limit) for the same game to be run on Mobile, Consoles and PCs. Games are usually coded in C#, although JavaScript is also supported.
It's scalability, along with extensive documentation and relative ease of use makes it the most used engine in the market.
Note that this tag is used mostly by Unity-chan (Real Name: Ootori Kohaku), the mascot character created by Unity Technologies Japan to represent the engine.