Resonance is the condition that exists when:

In an alternating current (AC) circuit, inductive reactance (the opposition to AC from a coil) and capacitive reactance (the opposition from a capacitor) act in opposite directions. Resonance is the specific state that occurs at the exact frequency where these two opposing reactances are perfectly equal in magnitude. Because they are 180 degrees out of phase, equal amounts of inductive and capacitive reactance will completely cancel each other out. When this happens at the resonant frequency, the circuit behaves as if it has no reactance at all, leaving only the circuit's true physical resistance to oppose the flow of current.