Isaac Newton called the colored bands produced by a prism a spectrum. The word entered optical science through Newton’s work in the 1660s and 1670s, from the Latin spectrum, meaning an appearance or apparition.
Newton used a prism to show that white sunlight is not made colorless by the glass. Instead, the prism separates the different colors already present in white light because each wavelength is refracted by a different amount. Violet bends more than red, spreading the colors into an elongated band.
The result is often casually called a rainbow, but that is not the most precise answer: a rainbow is a natural atmospheric spectrum formed by raindrops. “Prismatic colors” describes the appearance, while “spectrum” is Newton’s canonical term. Newton also demonstrated that the separated colors could be recombined with a lens and a second prism to produce white light.
He initially identified six main colors and later divided the band into seven by adding indigo, partly to associate the colors with musical notes. Modern physics uses spectrum more broadly for distributions of wavelengths or frequencies, including radio, infrared, visible, ultraviolet, and X-ray radiation.