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Games that try to simulate real-world activities (like driving vehicles or living the life of someone else) with as much realism as possible. Simulators generally require more study and orientation than arcade games, and the best simulators are also educational.

Involving mathematics

A game in which the player is not constrained to achieving specific goals and has a large degree of freedom to explore, interact with, or modify the game environment 

Games which have short playtimes and can usually be completed in one sitting.

Unity is a game engine for 2D, 3D, AR and more