A cross-platform application development framework, widely used for the development of GUI programs (in which case it is known as a widget toolkit), and also used for developing non-GUI programs such as console tools and servers.

Qt uses C++ with several non-standard extensions implemented by an additional pre-processor that generates standard C++ code before compilation. Qt runs on all major platforms, and has extensive internationalization support. Non-GUI features include SQL database access, XML parsing, thread management, network support and a unified cross-platform API for file handling.

Qt is most notably used in KDE, Opera, Google Earth, Skype, Qtopia, Photoshop Elements, VirtualBox and OPIE. Qt can also be used in several other programming languages; bindings exist for Ada (QtAda),[3] C# (Qyoto/Kimono),[4] Java (Qt Jambi),[5] Pascal, Perl, PHP (PHP-Qt), Ruby (RubyQt), and Python (PyQt).

A four-legged skeleton that makes contact with the floor using all four limbs.

A complex number made up of four geometric components.

A quaternion adds a fourth element to the [x, y, z] values that define a three-component-vector. A quaternion represents an axis in 3D space and a rotation around that axis.