The Scintillator reference article from the English Wikipedia on 24-Apr-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Scintillator

A scintillator is a device or substance that absorbs an electromagnetic wave of one wavelength and emits an electromagnetic wave of a different wavelength.

Common-life examples include

Scintillators are used in many physics research applications to detect electromagnetic waves or particles. There, a scintillator converts their wavelength (or energy - which is basically the same due to the wave-particle duality principle) to a wavelength which can be detected by inexpensive or easy to handle detectors.

In principle, Scintillators can have any aggregate state: gaseous, liquid or solid state. Most scintillators for practical use will come in solid state form.