Resonance

In physics, resonance is the tendency of a system to vibrate with increasing amplitudes at some frequencies of excitation. These are known as the system's "resonant frequencies" (or "resonance frequencies"). The resonator may have a "fundamental frequency," and any number of "harmonics".
An example of useful effect of resonant frequency is a guitar string which makes a characteristic noise when it is touched. The noise depends on how thick or slack the string is.
Another example can be seen in a playground swing. There is one specific rate at which you must push away a person on a swing to cause the swing to go high. This rate is the resonant frequency.
In earthquake engineering, on contrary, any possibility of a resonance is detrimental for the building structure.
The Kirkwood gap and other connections among orbits are caused by orbital resonance.