What property does fluorescence describe?

Fluorescence refers specifically to the phenomenon where certain substances emit light as a result of being exposed to radiation, usually ultraviolet light. When an object fluoresces, it absorbs energy from the radiation and then re-emits that energy as visible light. This property is particularly useful in various scientific fields, including biology and medicine, where fluorescent markers can be used to highlight specific structures or substances in diagnostic imaging or research.

The other options do not accurately describe fluorescence. A describes a thermal reaction, which is unrelated to light emission. C refers to a diagnostic technique, which may use fluorescence but does not define it. D discusses moisture absorption, which is a different physical property and not related to the emission of light. Thus, the defining characteristic of fluorescence is its direct relation to the emission of light upon exposure to radiation.