r45.
From the Resilience Alliance glossary—a useful source of definitions on systems-related topics—at www.resalliance.org/index.php/glossary . This widely-agreed definition is the same in essence as that given by systems analyst Paul A. Weiss: “A complex unit in space and in time, whose sub-units cooperate to preserve its integrity, its structure and its behaviour and tend to restore them after a non-destructive disturbance”, (1971), Hierarchically Organized Systems in Theory and Practice, p 99.