Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
I love this engine, even though I learned to make games with Unreal. I prefer Unity by far, its intuitive, versatile and professional. I just wish they had better tools for 2d platform games (like a grid) and some features that could help fellow 2d developers. But still a great engine.