The incoming solar radiation that reaches the earth and the atmosphere.
WRCC Climate and Weather Terms Glossary

Solar radiation incident upon a unit horizontal surface on or above the Earth's surface.
NASA Earth Observatory

Insolation - The solar radiation incident on a unit horizontal surface at the top of the atmosphere. It is sometimes referred to as solar irradiance. The latitudinal variation of insolation supplies the energy for the general circulation of the atmosphere. Insolation depends on the angle of incidence of the solar beam and on the solar constant. 
Atmospheric Chemistry Glossary