A platform from Microsoft for developing rich Internet applications on Windows and Mac computers. It was designed to compete with other multimedia platforms, including Adobe Flash/Flex, JavaFX and JavaScript and to broaden the scope for .NET developers, allowing them to run their .NET programs on the Mac (and Linux via Moonlight) rather than only Windows.

Delivered as a .NET-enabled plug-in for the Internet Explorer, Firefox and Safari browsers, Silverlight supports animation, vector graphics, MP3 and Windows Media formats including the VC-1 video codec. Silverlight programs can be written in Visual Basic, C#, Python and Ruby.

The Windows Interface - Everywhere
Code named "Windows Presentation Foundation/Everywhere" (WPF/E), Silverlight is the cross platform version of the Windows Presentation Foundation (WPF) user interface, first deployed in Windows Vista. Windows Mobile also supports Silverlight, and a Linux version is available from the Moonlight project. See VC-1, RIA, Moonlight and Windows Presentation Foundation.