Normalised Difference Vegetation Index (Rouse et al., 1974).

It is one of the first SVIs which was introduced to monitor vegetation (from Landsat 1) and over the years it has been widely used to estimate parameters such as e.g. Leaf Area Index, biomass, and fractional vegetation cover.
The index span from -1 to 1, with healthy vegetation usually occurring for values > 0.5. Values below that threshold often indicate overgrazed or unhealthy pasture lands.

The index formula is defined as:
(B8A - B4) / (B8A + B4)