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Typically a graphical style where there are one two colors. (A bit can be either on or off, so two options) Commonly black and white, but can be any combination depending on the style the creator is trying to accomplish. 

Only requires a single button (commonly the left mouse button or a single key on the keyboard) to operate the game's controls or interact with it.

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