Luminescent refers to the emission of light by a substance that is not caused by heat. This phenomenon can occur in various materials, including certain minerals, chemicals, and biological organisms. For example, some species of fireflies and jellyfish exhibit luminescence, producing light through biochemical reactions. There are different types of luminescence, such as fluorescence, where materials emit light when exposed to specific wavelengths, and phosphorescence, where the light continues to be emitted even after the excitation source is removed. Luminescent materials are used in various applications, including glow-in-the-dark products and LED technology.