It is standard to use the symbol $\Delta$ to indicate a change in a quantity between two points on a curve, two rows on a table, and so forth. For linear functions, we write slope = $\Delta y / \Delta x$; this notation is used all over the place in Physics and Chemistry.

But when looking at exponential functions, the quantity that relates naturally to $\Delta x = x_2-x_1$ is not $\Delta y = y_2 - y_1$, but rather $\bf{\frac{y_2}{y_1}}$. This suggests that it would be helpful to have some kind of notation to signify "multiplicative change," in analogy with using $\Delta$ to signify "additive change".

Is there such a symbol? Is it used in any textbooks?