Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal, which all are ultimately caused by Spontaneous emission. This distinguishes luminescence from incandescence, which is light emitted by a substance as a result of heating.
Thumbanil: Chemiluminescence after a reaction of hydrogen peroxide and luminol. This is an image from video https://youtu.be/8_82cNtZSQE. Image used with permission (CC BY-SA 4.0; Tavo Romann).