A model of a whole entity: when applied to human activity, the model is characterized fundamentally in terms of hierarchical structure, emergent properties, communication, and control. An observer may chose to relate this model to real-world activity. When applied to natural or man-made entities, the crucial characteristic is the emergent properties of the whole.
Defined by Checkland, P.B. (1999) Systems thinking, systems practice, Chichester, John Wiley.