(noun) - Technically speaking, "the" spectrum is what one sees when white light (i.e.: sunlight) is dispersed and refracted by a prism (or by something that acts like a prism, as sometimes happens to an innocent glass of water). For the best natural example of a spectrum, think of the colors in a rainbow.
The spectrum is something we all recognize (as opposed to white light, which one really doesn't see) because it's comprised of pure hues whose names we know. In fact, here's a mnemonic device (learned long ago in color theory class) which accurately describes the hues in the spectrum, in order. Just think of a man named "Roy G. Biv". If you break his name apart, you'll get the whole spectrum: R = red, O = orange, Y = yellow, G = green, B = blue, I = indigo, and V = violet.
Outside of art-speak, when one hears the word "spectrum" in conversation, the speaker is usually not referring to colors, hues, prismatic effects or the like. "Spectrum" is most commonly employed to mean "a wide range".
