Microsoft XNA (or XNA for short... stands for XNA is Not an Acronym) is a tool set designed for video game development. The XNA tool set makes it easier for game developers to create new video games without the hassle for having to write boilerplate code (repetitive code that has to be included in many sections of a game's programming).
The XNA tool set was first released on August 30, 2006 as XNA Game Studio Express. The tool set allows programmers to design games targeted for one of several platforms, including the PC, XBox 360, and Windows Phone 7 and also allows for integration with Visual Studio 2010.
Also see: Programming definitions