The atomic system drived by a strong coherent field at resonance or near resonance is excited and de-excited repeatedly at Rabi frequency. Inside a cavity or waveguide, the spectrum of the electromagnetic field modes is strongly modified for wavelengths that are comparable with or longer than the physical dimensions. Within a cavity of high quality factor (high-Q) the atoms are coupled to very few cavity modes and the radiation can survive long enough in the cavity to re-excite the atoms, making the spontaneous emission reversible and nonperturbative. Mesoscopic Multistability phenomenon results from the interaction of a beam of N Rydberg atoms, flying through a high-Q cavity and drived by a coherent field, with an excited single cavity mode at steady state. The term Multistability means that for certain input (driving) field intensity there may exist multiple values for the output (intracavity) field intensity.