UIL
Motif's declarative language for describing an interface's widget tree in a separate file rather than in C code.
The User Interface Language came from DEC's XUI toolkit and was carried over into Motif. A UIL file declares which widgets exist, how they nest, and what resources and callbacks they carry. The uil compiler turns it into a binary UID file, which the application loads at startup through Mrm, the Motif Resource Manager.
The point was to change layout, labels and language without recompiling the program — the same idea that later became Glade files for GTK, .ui files for Qt and XAML on Windows. UIL's syntax was verbose, though, and still tied tightly to Motif's widget names, so many developers built their interfaces directly in C or through Xresources instead.