The term Systems Theory is used in a broad way and is interpreted differently from different people. What is of interest here is the General System Theory, which goes back to the biologist and philosopher Ludwig von Bertalanffy, the mathematician Anatol Rapoport, the economist Kenneth Boulding, and others.
Bertalanffy argued that a system would be a „set of elements standing in interaction“ or „a complex of interacting elements“. Rapoport claimed that a system would be „a whole that functions as a whole by virtue of the interaction of its parts“.
„I seem to have come to much the same conclusion as you have reached, through approaching it from the direction of economics and the social sciences rather than from biology–that there is a body of what I have been calling „general empirical theory“, (or general systems theory, in your excellent terminology) which is of wide applicability in many different disciplines. I am sure there are many people all over the world who have come to essentially the same position that we have, but we are widely scattered and do not know each other, so difficult is it to cross the boundaries of the disciplines“ wrote Kenneth Boulding to Ludwig von Bertalanffy in 1953 after reading a GST article written for Scientific Monthly.