A system is said to be critical if its state changes dramatically given some small change in an input value to a control parameter. Once the system reaches its critical point even the smallest perturbation can have major consequences, resulting in uncontrollability. As the system becomes more critical its eventual collapse becomes greater and its eventual transformation becomes more inevitable but less predictable. Beyond the critical point there is a runaway affect, as a tipping point has been passed and the system moves into a phase transition; irreversibly moving into a new state. At this stage the system becomes extremely nonlinear, cause and effect break down almost completely, massive direct interventions within the system can have very negligible effects. An example of this might be a financial crisis, where once a major institution goes under there is a loss of confidence with an ensuing bank run(the tipping point) after which the  government can put billions into the market, buying up assets and only have a negligible effect on the price(highly nonlinear).