According to the Oxford English Dictionary:
a. The coloured band into which a beam of light is decomposed by means of a prism or diffraction grating. Also, a dark band containing bright lines produced similarly; such a (coloured or dark) band, or the pattern of lines in it, as characteristic of the light source; hence, the pattern of absorption or emission of light or other electromagnetic radiation over any range of wavelengths exhibited by a body or substance.
The first known use of the word "spectrum" was:
1672 I. Newton in (Royal Soc.) 6 3076 Comparing the length of this coloured Spectrum with its breadth, I found it about five times greater.