The Interactive Glossary: Defining the Net
Flash

Flash is an animation and authoring program specifically developed for use on the Web by Adobe, Inc. Viewing Flash animations requires a plug-in, a software program specifically designed to work with an existing program, in this case, your web browser. Flash plugs into your web browser and plays animations embedded on a web page.
Internet Explorer, Firefox and Safari all support Flash and recent versions come bundled with the Flash player. If you don't already have it, you can download it for free from the Adobe website.