A set of concepts within a system and the relationships among those concepts (e.g., view and viewpoint). A system model describes, using one diagram type (such as in Object-Process Methodology (OPM)) or several diagram types (such as in Systems Modeling Language (SysML)) the various aspects of the system. The model might include its requirements, behavior, structure, and properties. In addition, a system model is accompanied by a set of definitions for each concept. Sometimes, system concept models are defined using an entity relationship diagram, an object-process diagram (OPD), or a Unified Modeling Language (UML) class diagram.