Graphical or textual languages that are used to build models by describing the qualities with which the models are concerned. A model represents something and includes some but not all qualities of what it represents. For example, an architect's model of a new development area concentrates on the physical appearance, while an aircraft designer's wind-tunnel model concentrates on aerodynamic qualities. When models are used in systems and software engineering, their important qualities are structure, behavior, communication, and requirements.