Systems theory refers to a study of the relationship of parts in their context, emphasizing their unity and their relationship to each other. It is applied to biology, medicine, and other fields and is a basis for family systems therapy.
Systems theory is also a term used in Psychopathology that refers to a framework for viewing human behavior through three (3) distinct frames of reference or "systems”—biological, psychological, and social; systems theory does not attempt to explain human behavior
Other /More definition:
Systems theory is a term used in the study of groups that refers to a general theoretical approach which assumes the groups are systems - collections of individual units that combine to form an integrated, complex whole.
Systems theory also refers to the viewpoint that all levels of an organization in any entity are linked to each other hierarchically and that change in any level will bring about change in other levels.