"The change of the output is not proportional to the change of the input."
Study of nonlinear and chaotic systems.
"Most systems are inherently nonlinear in nature."
"They may appear chaotic, unpredictable, or counterintuitive."
"A set of simultaneous equations in which the unknowns appear as variables of a polynomial of degree higher than one or in the argument of a function which is not a polynomial of degree one."
"Yes, systems can be defined as nonlinear, regardless of whether known linear functions appear in the equations."
"Nonlinear systems are commonly approximated by linear equations (linearization)."
"Interesting phenomena such as solitons, chaos, and singularities are hidden by linearization."
"Some aspects of the dynamic behavior of a nonlinear system can appear to be counterintuitive, unpredictable, or even chaotic."
"This nonlinearity is one of the reasons why accurate long-term forecasts are impossible with current technology."
"Some authors use the term nonlinear science for the study of nonlinear systems."
"This term is disputed by others."
"Using a term like nonlinear science is like referring to the bulk of zoology as the study of non-elephant animals."