Flex SDK stands for Flex Software Development Kit. It is a programming framework and SDK developed by Adobe Systems for building rich web applications. Flex SDK allows developers to create applications using the Flex framework, which is based on Adobe Flash technology.

The Flex SDK includes a compiler, libraries, and other tools necessary for developing Flex applications. It supports various programming languages such as ActionScript (a scripting language similar to JavaScript) and MXML (a markup language used for defining user interface components and layouts).

Flex SDK provides a set of components and controls that can be used to create interactive and responsive user interfaces. It also supports data binding, styling, and other advanced features for building complex applications.

Flex SDK can be used with different integrated development environments (IDEs) such as Adobe Flash Builder and Adobe Animate. It allows developers to build cross-platform applications that can run on different operating systems and devices.

However, it is worth noting that Adobe announced the end-of-life for Flex in 2011 and stopped further development and support for the Flex framework in 2013. Although Flex is still in use, its popularity has declined in favor of other web development technologies such as HTML5 and JavaScript frameworks like React and Angular.