A complex system is one in which a variety of elements interact and adapt over time, giving rise to unpredictable and sometimes surprise behaviour. These relationships among the elements and their adaptive processes are usually nonlinear and non-hierarchical, meaning local actions can influence distant elements, and vice versa. While specific models and approaches can be used to analyze, understand and even predict the behaviour of certain complex systems, overall the system often remains largely unpredictable and emergent.

See also: complexity, systems theory, systems thinking, systems change