Damping is an influence within or upon an oscillatory system that has the effect of reducing, restricting or preventing its oscillations. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples include viscous drag in mechanical systems, resistance in electronic oscillators, and absorption and scattering of light in optical oscillators. Damping not based on energy loss can be important in other oscillating systems such as those that occur in biological systems. The damping of a system can be described as being one of the following: Overdamped: The system returns (exponentially decays) to equilibrium without oscillating. Critically damped: The system returns to equilibrium as quickly as possible without oscillating. Underdamped: The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero. Undamped: The system oscillates at its natural resonant frequency (ωo). For example, consider a door that uses a spring to close the door once open. This can lead to any of the above types of damping depending on the strength of the damping. If the door is undamped it will swing back and forth forever at a particular resonant frequency. If it is underdamped it will swing back and forth with decreasing size of the swing until it comes to a stop. If it is critically damped then it will return to closed as quickly as possible without oscillating. Finally, if it is overdamped it will return to closed without oscillating but more slowly depending on how overdamped it is. Different levels of damping are desired for different types of systems.