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Oscillation
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<p>Oscillation is the repetitive or <a href="/facts/Periodic_function/Sm8lJQsF">periodic</a> variation, typically in <a href="/facts/Time/HceJKUGB">time</a>, of some measure about a central value (often a point of <a href="/facts/Mechanical_equilibrium/N2XKaK6s">equilibrium</a>) or between two or more different states. Familiar examples of oscillation include a swinging <a href="/facts/Pendulum/D2hQXwlI">pendulum</a> and <a href="/facts/Alternating_current/qxICS3xa">alternating current</a>. Oscillations can be used in physics to approximate complex interactions, such as those between atoms. </p><p>Oscillations occur not only in mechanical systems but also in <a href="/facts/Dynamic_system/782gREl5">dynamic systems</a> in virtually every area of science: for example the beating of the <a href="/facts/Human_heart/fF4KSGdk">human heart</a> (for circulation), <a href="/facts/Business_cycle/etH0t44s">business cycles</a> in <a href="/facts/Economics/HYctA2Dn">economics</a>, <a href="/facts/Predator%25E2%2580%2593prey/JEGdrX3f">predator–prey</a> population cycles in <a href="/facts/Ecology/YE8upuJU">ecology</a>, geothermal <a href="/facts/Geyser/cjXV4fCP">geysers</a> in <a href="/facts/Geology/WGvVRIYF">geology</a>, vibration of strings in <a href="/facts/Guitar/QIJdTYa6">guitar</a> and other <a href="/facts/String_instrument/srW2H8dV">string instruments</a>, periodic firing of <a href="/facts/Nerve_cell/G7CyTVdH">nerve cells</a> in the brain, and the periodic swelling of <a href="/facts/Cepheid_variable/WQXaAFmW">Cepheid variable</a> stars in <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>. The term <i><a href="/facts/Vibration/sGgDqG71">vibration</a></i> is precisely used to describe a mechanical oscillation. </p><p>Oscillation, especially rapid oscillation, may be an undesirable phenomenon in <a href="/facts/Process_control/tmjQhchm">process control</a> and <a href="/facts/Control_theory/aIuERpn6">control theory</a> (e.g. in <a href="/facts/Sliding_mode_control/218PP4uw">sliding mode control</a>), where the aim is convergence <a href="/facts/Stability_theory/Su5sHnxs">to stable state</a>. In these cases it is called chattering or flapping, as in <a href="/facts/Valve/EwgqAwlJ">valve</a> chatter, and <a href="/facts/Route_flapping/uHoY92n9">route flapping</a>. </p>