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Direct non-linear proportion
A relationship where $y$ is proportional to a positive power of $x$: $y\propto x^n$ with $n>0$, so $y=kx^n$.
Negative proportion
A linear proportional relationship with a negative constant of variation: $y=-kx$ where $k>0$.
Proportionality constant
A non-zero constant, usually called $k$, that links two proportional variables (also called the constant of variation).