A structured set of objects and / or attributs. The objects and attributes consists of components or variables (i.e. phenomena which are free to assume variable magnitudes) that exhibit discernible relationships which one another and operate together as a complex whole according to some observation pattern.

Ref.: Kirkby. M. J.; Naden, P. S.; Burt, T. P. & Butcher, D. P. (1992): Computer Simulation in Physical Geography. Sec. Edition, John Wiley & Sons, Chichester, New York, Brisbane, Toronto, Singapore.