normalized difference vegetation index
An index, or scale, of vegetation conditions based on differences in the amount of visible and near-infrared light reflected from plants on Earth's surface. A vegetation index is a gauge of plant health, productivity, and density.
Plant leaves absorb visible light for use in photosynthesis, and they reflect near-infrared light. A light-measuring sensor looking at a healthy plant sees very little reflected visible light and lots of near-infrared light. The more healthy green leaves a plant has, the stronger the difference between the amount of visible and near-infrared light it reflects, and the higher the vegetation index. An unhealthy plant will have fewer green leaves, resulting in a lower score on the vegetation index. Source: NASA (http://earthobservatory.nasa.gov/Glossary)