Self-organized criticality is a property of dynamical systems that have a critical point as an attractor. Their macroscopic behavior thus displays the spatial or temporal scale-invariance characteristic of the critical point of a phase transition, but without the need to tune control parameters to a precise value, because the system tunes itself as it evolves towards criticality. The concept was put forward by Per Bak, Chao Tang and Kurt Wiesenfeld, and is considered to be one of the mechanisms by which complexity arises in nature.
Images from: "Role of Network Science in the Study of Anesthetic State Transitions" and "Tuning Pathological Oscillations with EEG Neurofeedback and Self-Organized Criticality"