i) Diffuse radiation: It is the solar radiation which is scattered by the particles in earth’s atmosphere and this radiation does not have any unique direction.
ii) Beam radiation: Solar radiation along the line joining the receiving point and the sun is called beam radiation. This radiation has unique direction.
iii)Insolation: It is defined as the total solar radiation energy received on a horizontal surface of unit area on the ground in unit time.
iv) Solar constant: It is defined as the energy received from the sun per unit time on unit surface area perpendicular to the direction of propagation of solar radiation at the top of earth’s atmosphere when earth is at its mean distance from the sun.
OR
It is the average radiation received outside the earth’s atmosphere