A general definition of Flash can be found on Wikipedia (http://en.wikipedia.org/wiki/Adobe_Flash):
Adobe Flash (previously known as Macromedia Flash) is a multimedia platform originally acquired by Macromedia and currently developed and distributed by Adobe Systems. Since its introduction in 1996, Flash has become a popular method for adding animation and interactivity to web pages. Flash is commonly used to create animation, advertisements, and various web page Flash components, to integrate video into web pages, and more recently, to develop rich Internet applications.
Flash is able to manipulate vector and raster graphics, supports streaming of audio and video, and comes with a scripting language called ActionScript. Flash content is either authored with dedicated authoring software such as the "Adobe Flash Professional multimedia authoring program" or it can be put together programmatically with libraries such as "Ming".
Files in the Flash format usually have the .swf
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