Answer: It is the arrangement of electromagnetic radiations on the basis of their wavelengths or frequencies.
or
E= energy
= wavelength
= frequency
As energy is inversely related to wavelength and directly related to frequency, thus radiations with high wavelengths have low energy and low frequency and vice versa.
In order from highest to lowest frequency , the radiations of the spectrum are gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.