The air mass coefficient defines the direct optical path length through the Earth's atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith.
The air mass coefficient can be used to help characterize the solar spectrum after solar radiation has traveled through the atmosphere.
The air mass coefficient is commonly used to characterize the performance of solar cells under standardized conditions, and is often referred to using the syntax "AM" followed by a number.
"AM1.5" is almost universal when characterizing terrestrial power-generating panels.