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

The normalized difference vegetation index is a simple, but effective index for quantifying green vegetation. It is a measure of the state of vegetation health based on how plants reflect light at certain wavelengths.
The value range of the NDVI is -1 to 1. Negative values of NDVI (values approaching -1) correspond to water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while healthy vegetation usually occurring for values >= 0.5.

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