What type of material emits light by fluorescence?

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Fluorescence is a phenomenon where certain materials absorb light or other electromagnetic radiation and then re-emit it almost immediately, typically in a different wavelength. Phosphors are specially designed materials that exhibit this behavior. When exposed to a source of energy, such as ultraviolet light, phosphors can absorb the energy and then release it as visible light. This ability to emit light through fluorescence is a key property of phosphors, making them widely used in various applications such as fluorescent lights, glow-in-the-dark materials, and certain types of displays.

While other materials like dyes can also exhibit fluorescence, phosphors are more specifically associated with strong and sustained fluorescent properties. Minerals might fluoresce depending on their composition, but they aren't specifically defined as fluorescent materials. Incandescent materials emit light through thermal radiation, which is a different process altogether. Thus, phosphor stands out as the correct answer due to its distinctive and direct relation to the phenomenon of fluorescence.