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The Collaborative International Dictionary of English v.0.48 (gcide)

Fluorescence \Flu`o*res"cence\, n. [From {Fluor}.] (Chemistry,
Optics)
A luminescence emitted by certain substances due to the
absorption of radiation at one wavelength, and the almost
instantaneous re-emission of radiation at another, usually
longer wavelength. The re-radiation stops almost as soon as
the incident radiation is halted, thus distinguishing this
phenomenon from {phosphorescence}, in which re-radiation of
light may continue for some time after the incident radiation
is halted.

Note: The color of the radiated light typically differs from
the apparent color of the material, as when green
crystals of fluor spar afford blue reflections. It is
due not to the difference in the color of a distinct
surface layer, but to the power which the substance has
of modifying the light incident upon it, by first
absorbing the light to achieve an excited state, and
then radiating light to resume the ground energy level.
The light emitted by fluorescent substances is in
general of longer wavelength than the incident light.
The radiation can also be induced by ionizing radiation
which is not electromagnetic, such as alpha or beta
rays, and cathode rays. This property is possessed by
fluorspar, uranium glass, sulphide of calcium, and many
other substances. It finds use in analytical
instruments to detect or measure radiation, and in some
commercial applications.
[1913 Webster PJC]