Nonlinear response refers to the behavior of a system or material that does not exhibit a linear relationship between input and output. In other words, the response of the system is not proportional to the stimulus applied to it. Nonlinear responses are often observed in complex systems such as biological systems, materials, and mechanical structures. Researchers in this area study the non-linear dynamics and behaviors of these systems, aiming to understand and predict their responses to various stimuli. This research has applications in various fields such as physics, engineering, biology, and economics.