In mathematics, the elasticity or point elasticity of a positive differentiable function f of a positive variable (positive input, positive output)[1] at point a is defined as[2] using functions and their derivatives

meaning in words: It is thus the ratio of the relative (percentage) change in the function's output
with respect to the relative change in its input
, for infinitesimal changes from a point
.
In mathematics, the elasticity or point elasticity of a positive differentiable function f of a positive variable (positive input, positive output)[1] at point a is defined as[2] using logarithms

meaning in words: it is the ratio of the infinitesimal change of the logarithm of a function with respect to the infinitesimal change of the logarithm of the argument.