Fluorescence is the spontaneous emission of light shortly after a material is excited by light. The photons emitted are generally lower in energy than those previously absorbed. Physical systems in which fluorescence occurs are called fluorophores. Fluorescent substances that are used for staining are called fluorochromes or fluorescent dyes. If the excitable material is part of an organism, it is also referred to as biofluorescence (analogous to bioluminescence). If an object is fluorescent by itself, i.e. without having to be stained, it is referred to as autofluorescence or self-fluorescence.

Source: Wikipedia