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nonlinearity/step function

In mathematics and in the context of cybernetics, nonlinearity refers to a relationship between variables that does not form a straight line in a right - angled, Cartesian system of coordinates. Its opposite is linearity (lineality / linearity), where change in one variable effects a proportional change in another variable. Besides continuous nonlinear relationships, there exist discontinuous nonlinear relationships that are called step functions in mathematics. These form the mathematical counterpart of what is denoted in systems theory as second-order change (change, first and second order).
Credit

Definition extracted with permission from Simon, Fritz, et al, Family Process, Inc.: Language of Family Therapy: A Systemic Vocabulary and Source Book (Family Process Press Series)