Luminescence was the spontaneous emission of light from an object. Lifeforms capable of producing luminescence were further described as being bioluminescent. Objects and lifeforms with this ability were colloquially know to "glow in the dark".
When the amnesiac Data began investigating the mysterious illness that was affecting the Barkonians that he befriended in 2370, he coated a piece of cloth with the liquid used in lamps, which he demonstrated would become luminescent when exposed to an energy source. With it, he was able to determine that a metallic pendant they had appeared to be an energy source. When Talur inquired where the pattern of light causing the luminescence was coming from, Data explained that he believed that a stream of particles was emanating from the pendant and hitting the cloth. (TNG: "Thine Own Self")
In describing the graviton ellipse encountered by USS Voyager and entered by the Delta Flyer in 2376, Commander Chakotay described its interior to be "very calm", with "no gravimetric distortions," while further describing that the EM activity was "creating a natural luminescence." Tom Paris further described the luminescence as "mood lighting", adding that "it's the perfect romantic getaway." (VOY: "One Small Step")
Bioluminescence[]
A Dakalan bug
Bioluminescence was the ability of some lifeforms to produce and emit light. On Earth, various plants, fungi, and other small lifeforms, such as fireflies, gave off light.
In 2151, Enterprise NX-01 visited the planet of Dakala and the crew saw a "bug" emitting light. (ENT: "Rogue Planet")
At the first Annual Voyager Science Fair on the USS Voyager, in 2376, Mezoti presented a transparent box containing soil and a colony of luminescent Teirenian ants. She had infused the soil with a blue ion dye so the insects would be easier to see. (VOY: "Child's Play")
Risa had subterranean gardens in which all the plants were luminescent. (ENT: "Two Days and Two Nights")
Gedi Prime contained waterfalls glowing with bioluminescence, a popular tourist destination. (VOY: "Drive")