A server of connections to X terminals in a distributed network that uses the X Window System. From the terminal user's point-of-view, the X server may seem like a server of applications in multiple windows. 

An open, cross-platform, client/server system for managing a windowed graphical user interface in a distributed network. 

A collective term said to stand for a number of things including "X as a service," "anything as a service" or "everything as a service".  XaaS is the essence of cloud computing.

An open standard XML-based language designed to express security policies and access rights to information for Web services, digital rights management (DRM), and enterprise security applications. 

Xamarin is a cross platform mobile app development software with a C# shared codebase, which developers can use to write native applications for iOS, Android and Windows Mobile.

Mono is an open source software developed by Xamarin to build a .NET compatible tools like C# compiler and Common Language Runtime. Mono is developed not only to run Microsoft .NET applications cross-platform but it can also run on many software systems including Android, most Linux distributions, BSD, OS X, Windows, Solaris, and even some game consoles such as PlayStation 3, Wii, and Xbox 360.

Microsoft's XML-based language for creating a rich GUI, or graphical user interface. XAML supports both vector and bitmap types of graphics, as well as rich text and multimedia files.

XAMPP stands for Cross-Platform (X), Apache (A), MySQL (M), PHP (P) and Perl (P), is a free and open source cross-platform web server solution stack package. It is simple and lightweight makes it extremely easy for developers to create a local web server for testing purposes. XAMPP is also cross-platform, which means it works equally well on Linux, Mac and Windows.

Xapian is an Open Source full text Search Engine Library, released under the GPL. It's written in C++, with bindings to allow use from Perl, Python, PHP, Java, Tcl, C#, Ruby, Lua, Erlang and Node.js (so far!) Xapian is a highly adaptable toolkit which allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also supports a rich set of boolean query operators.

Xbox One is a video game console developed and marketed by Microsoft. Announced on May 21, 2013, it is the successor to the Xbox 360 and is the third console in the Xbox family. The console features an AMD processor built around the x86-64 instruction set and  includes a newly upgraded Kinect motion sensing peripheral.

An XML-based computer language for the electronic transmission of business and financial data. The goal of XBRL is to standardize the automation of business intelligence (BI).

A specification language for writing security checklists, benchmarks and related types of documents. XCCDF documents are expressed in XML and can be validated with an XML validating parser.

XCTest is the new testing framework and a succesor of OCUnit, introduced with Xcode 5. XCTest offers better integration with Xcode and lays the foundation for future improvements to Xcode testing capabilities. Many of the functions in XCTest are similar to the ones in OCUnit.  

A network architecture for multicast streaming media transmissions. Streaming media enables live viewing of a transmission as compared to processes that require  files to be completely transferred before their data can be viewed.

Citrix XenServer is a free open source virtualization platform for managing cloud, server and desktop virtual infrastructures. XenServer is based on the Xen Project™ hypervisor, a bare metal virtualization paltform used by XenServer to deliver near native application performance for x86 workloads in an Intel and AMD environment.

Xero is an online cloud based accounting software for small and medium-sized businesses, as well as for personal finance.