Resonance (LC)
The frequency where an inductor's and capacitor's reactances cancel: f0 = 1/(2π√(LC)). Energy sloshes between the magnetic and electric field with the source only topping up losses — tiny inputs build large swings.
In practice
Wanted: tuned circuits, crystal oscillators, wireless charging. Unwanted: the surprise peak in your supply filter, ringing on switching edges. Same equation either way — if there's an L and a C, there's an f0, whether you planned one or not.