Is there a term for this form of equation or ratio?

Question

I am referencing and explaining the use of the Normalized Difference Vegetation Index (NDVI) in my work and am lost for a term that describes the form of the equation. It takes the form

$$ NDVI = \frac{NIR - Red}{NIR + Red} $$

Equations of this form pop up in many other fields of math and science and if I remember correctly there is some term for the form, specifically a ratio in the form

$$ \frac{a - b}{a + b} $$

I would assume the term would be something like "normalized difference" or "difference ratio", though a quick google search provides no general results.

Does such a term exist for this type of equation or ratio?

Answer 1

The wikipedia page https://en.wikipedia.org/wiki/Relative_change_and_difference calls an expression of the form $$\left|\frac{x-y}{f(x,y)}\right|,$$ where $f(x,y)$ is a real-valued function, a relative difference.

The wikipedia page that you linked calls NDVI a difference-sum ratio.