A relationship where the entities do not remain proportional to each other.
For example, if one member of the relationship doubles, the other might not.
1. All relationships that aren't non-linear are linear.
2. Usage example: Acccording to Richard Lindzen, CO2's relationship to atmospheric warming is non-linear:
"the impact of CO2 is nonlinear in the sense that each added unit contributes less than its predecessor. For example, if doubling CO2 from its value in the late 19th — from about 290 parts per million by volume (ppmv) to 580 ppmv — causes a 2 percent increase in radiative forcing, then to obtain another 2 percent increase in radiative forcing we must increase CO2 by an additional 580 ppmv rather than by another 290 ppmv.
…. The easiest way to understand this is to consider adding thin layers of paint to a pane of glass. The first layer cuts out much of the light, the next layer cuts out more, but subsequent layers do less and less because the painted pane is already essentially opaque."