Software from Citrix that eliminates the permanent installation of applications in the user's machine. It provides "local application delivery" by streaming the application as needed to the Citrix Receiver client in the user's machine. See application streaming.

XenApp also provides "hosted application delivery," which is a centralized, terminal-based architecture for Windows and Unix servers. Also known as a "thin client" environment, it is the reason the company was founded. Using Citrix's ICA protocol, which governs keyboard, mouse and screen transfer, a user's machine is turned into an input/output (I/O) terminal to the server, which runs any and all applications that might normally run on a desktop PC. In lieu of PCs, low-cost ICA terminals can also be used. See ICA.

XenApp and Terminal Services
XenApp relies on Microsoft's Terminal Services, which lets multiple users simultaneously share one copy of an application running in a Windows server. Although Terminal Services can be used without XenApp to provide multiuser computing, XenApp adds a raft of enhancements for central control, printers, server farms, handheld devices, scalability and policy management. XenApp also enables a wider variety of platforms to be used as terminals. Another advantage over Terminal Services without XenApp is speed on wide area networks, because XenApp moves much of the user interface graphics rendering into the client machine eliminating bandwidth-intensive graphics traveling over the network.

Interestingly, the foundation of Microsoft's Terminal Services itself is based on Citrix software. In 1997, Citrix licensed its MultiWin technology to Microsoft and it became the underlying Terminal Services foundation. Citrix XenApp was formerly Citrix Presentation Server and originally Citrix MetaFrame. See thin client, Terminal Services, centralized processing and Citrix XenDesktop.

Unix X Window System
In Unix, X Window provides the shared terminal services. XenApp sets up an X Window partition in the server for each user and converts between X Window and ICA. See X Window.