nonlinearity

1. Property of chaotic systems, characterized by approximation, random behavior, and unpredictability. It is the feature of natural (real world) or other systems which cannot be decomposed into parts and reassembled into the same thing, and do not change in proportion to a change in an input.

2. Situation where the relationship between variables is not simply static or directly proportional to the input, but instead is dynamic and variable. In a factory, for example, the per unit cost may decrease instead of staying constant as the output level rises (due to economies of scale), and may start to rise after the optimum output level is reached (due to diseconomies of scale).