Luminescent describes something that gives off a soft shining light without being on fire and without being hot. A luminescent fish, a luminescent sign, luminescent paint on a wall. The noun is luminescence; this is the adjective, the 'what kind of thing is it?' word. In shops and on packets you will see it next to glow-in-the-dark toys and safety marks. Though actually, everyday English still prefers glowing or glow-in-the-dark; luminescent sounds more careful, more like science or a label. People also stretch fluorescent to cover this, but a fluorescent light in an office is a different, harsher thing — a tube that needs electricity — not the same as a fish that makes its own soft light.