The term 'Spectrum Band' refers to the colors of the visible light spectrum, which are INDIGO (6 letters), RED (3 letters), BLUE (4 letters), and VIOLET (6 letters). These colors are often used to represent the different wavelengths of light that can be seen by the human eye. The colors of the spectrum band are arranged in order of increasing wavelength, with INDIGO having the longest wavelength and VIOLET having the shortest.