Nonlinearity refers to a relationship in which changes in one variable do not produce proportional changes in another. In simple terms, if you double one input, the output may not necessarily double. This concept is common in various fields, including mathematics, physics, and economics, where systems can behave unpredictably due to complex interactions. In contrast to linear systems, where outputs are directly proportional to inputs, nonlinear systems can exhibit behaviors like chaos and sudden shifts. Examples of nonlinearity can be found in weather patterns, population dynamics, and even in neural networks, where small changes can lead to significant and unexpected outcomes.