Fluorescence is the process by which an atom absorbs light at one wavelength and immediately re-emits it at another wavelength. In a comet the CO+ ions absorb the light of the Sun and then re-emit it by fluorescence as blue light, giving the colour to the tail of ions (carbon monoxide CO is, after water, the most abundant element in comets).
In fluorescent lamps (neon tubes), a gas emits ultra-violet light when an electrical discharge passes through it. A phosphor coating absorbs this ultraviolet light and transforms it by fluorescence into visible light.
See also phosphorescence.