In the realm of electricity, a circuit is said to be in resonance if the inductive and capacitive reactances of the circuit cancel out, leaving only the resistance. This state only occurs at one frequency called the resonant frequency. This frequency can be determined by using the following formula:
Frequency = 1 / 2pi * sqrt (capacitance * inductance)
If the capacitance is given in farads, and the inductance in henrys, the frequency will come out as hertz. This frequency not only is the frequency that the circuit will pass with the least impedance, it is also the fundmental frequency the circuit will assume if the circuit is charged, then allowed to discharge through itself.