Luminescence is the "spontaneous emission of radiation from an electronically excited species (or from a vibrationally excited species) not in thermal equilibrium with its environment", according to IUPAC definition. A luminescent object is emitting "cold light", in contrast to "incandescence", where an object only emits light after heating. Generally, emission of light is due to the movement of electrons between different energy levels within an atom after excitation by external factors. However, the exact mechanism of light emission in "vibrationally excited species" is unknown, as seen in sonoluminescence.

There are various ways to make an object emit light, such as by light (photoluminescence), chemical reactions (chemiluminescence), electrical energy (electroluminescence), ultrasound vibrations (sonoluminescence), and by ionising radiation (radioluminescence).

The dials, hands, scales, and signs of aviation and navigational instruments and markings are often coated with luminescent materials in a process known as "luminising".