Spontaneous emission is the process by which an excited atom or molecule releases excess energy as a photon without any external stimulation.
Fluorescence: The emission of light by a substance that has absorbed photons and re-emits them at longer wavelengths.
Phosphorescence: Similar to fluorescence but with a delayed emission of light after the excitation source is removed.
Radiative Decay: The process where an excited state decays back to its ground state by emitting electromagnetic radiation.