UsiXML is an XML

Extensible Markup Language

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

-based markup language for defining user interface

User interface

The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...

s on computers.

UsiXML is a specification language for user interface design. It allows the designer to describe a user interface at different levels of abstraction. In other words you can specify a UI in terms of functionality (task analysis

Task analysis

Task analysis is the analysis of how a task is accomplished, including a detailed description of both manual and mental activities, task and element durations, task frequency, task allocation, task complexity, environmental conditions, necessary clothing and equipment, and any other unique factors...

), object it manipulates, or, in a more concrete way, you can draw user interfaces.

The UsiXML language is currently being submitted for a standardisation plan to the W3C

World Wide Web Consortium

The World Wide Web Consortium is the main international standards organization for the World Wide Web .Founded and headed by Tim Berners-Lee, the consortium is made up of member organizations which maintain full-time staff for the purpose of working together in the development of standards for the...

.

Another work with the same purpose is UIML

UIML

UIML is an XML language for defining user interfaces on computers.Basically UIML tries to reduce the work needed to develop user interfaces. It allows you to describe the user interface in declarative terms and abstract it...

.

Tool support

There are plenty of tools that can be found for UsiXML.
They include: a translator from UsiXML specification to Flash (FlashiXML), a tool for drawing/sketching user interfaces (SketchiXML), a tool for task analysis (idealXML).