From AMS Glossary
A numerical value used to predict or assess vegetative characteristics such as plant leaf area, total biomass, and general health and vigor of the surface vegetation.
Vegetation indices are usually derived from multispectral remote sensing observations. Since growing plants strongly reflect the wavelengths of light in the near-infrared, combinations of measurements in the near-infrared and visible-red portions of the spectra are used to generate a variety of different indices. Perhaps the most common vegetation index is the Normalized Difference Vegetation Index (NDVI), defined as
where NIR and RED are the radiance or reflectance measured in the near-infrared and red portions of the spectrum, respectively. This, and other indices, can be adapted to a variety of remote sensing satellites and instruments.