Systems theory or systems science is the interdisciplinary study of systems in general, with the goal of discovering patterns and elucidating principles that can be discerned from and applied to all types of systems at all nesting levels in all fields of research. It can reasonably be considered a specialization of systems thinking or as the goal output of systems science and systems engineering, with an emphasis on generality useful across a broad range of systems (versus the particular models of individual fields).
A system may be defined as bounded set of interrelated or interacting elements. This definition accommodates passive structures (e.g. a necklace, or the Dewey Decimal System). However, most system theorists focus on activity systems in which structures/components interact in behaviours/processes. And many focus on social activity systems in particular.
An early focus of system theory was systems that maintain themselves in consistent or viable state through feedback. These self-regulating systems are found in nature, including the physiological systems of our body, in local and global ecosystems, and in climate and also in human learning processes (from the individual on to international organizations like the UN).
The term "general system theory" originates from Bertalanffy's general system theory (GST). His ideas were picked by others including Kenneth E. Boulding, William Ross Ashby and Anatol Rapoport working in mathematics, psychology, biology, game theory and social network analysis.
Sociological systems thinking started earlier, in the 19th century. In: Bertrand Badie et al. (eds.), International Encyclopedia of Political Science. Sage New York, Stichweh states: "... Since its beginnings the social sciences were an important part of the establishment of systems theory... the two most influential suggestions were the comprehensive sociological versions of systems theory which were proposed by Talcott Parsons since the 1950s and by Niklas Luhmann since the 1970s." References include the action theory of Talcott Parsons and the social systems theory of Niklas Luhmann.