XUL (/ˈzuːl/ ZOOL, ) XUL stands for XML User Interface Language. XUL is a markup language that is developed by the Mozilla Project. XUL is a user interface markup language implemented as an XML dialect; it allows for graphical user interfaces to be written in a similar manner to Web pages.

XUL can be used to write cross-platform applications such as Mozilla Firefox, where it is interpreted by the layout engine known as Gecko which renders Firefox's user interface and Web page display.

Read more about XUL:  Design, XUL Documents, XUL Applications, The Name, Example, Similar Languages