Resonant frequency is the natural frequency at which a system tends to oscillate with maximum amplitude when disturbed from equilibrium. Every physical system that can store and transfer energy between two forms (such as kinetic and potential energy in mechanical systems, or electric and magnetic energy in electrical systems) has one or more resonant frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations because the system stores energy efficiently.

Understanding resonance is crucial in engineering and physics because it can be both beneficial and destructive. Engineers design systems to either exploit resonance (as in musical instruments and radio tuners) or avoid it (as in buildings during earthquakes). The resonant frequency depends on the system's physical properties: in mechanical systems, it's determined by mass and stiffness; in electrical LC circuits, it's determined by inductance and capacitance.