Definition extracted with permission from Simon, Fritz, et al, Family Process, Inc.:Language of Family Therapy: A Systemic Vocabulary and Source Book (Family Process Press Series)
The most general definition of system [Greek systema, a composite thing] is the ordered composition of (material or mental) elements into a unified whole. The various fields of systems research concentrate on differing aspects or perspectives of the elements and systems. General systems theory, like cybernetics, concerns itself with the functions and structural rules valid for all systems, irrespective of their material constitution. The premises of systems theory are based on the insight that a system as a whole is qualitatively different, and "behaves" differently, from the sum of the system's individual elements. In the framework of family therapy, the application of the term "system" is identical to its application in the field of cybernetics. This concept takes account of system features like feedback, the processing and storage of information, adaptability, the ability for self-organization, and the development of strategies for the system's own behavior.