Resonance is the condition that exists when:

Resonance in an electrical circuit occurs when the opposing forces of inductive reactance and capacitive reactance perfectly cancel each other out. Because inductors cause voltage to lead current and capacitors cause voltage to lag current, their respective reactances are opposite in sign or phase. When these two reactances are equal in magnitude, their effects completely neutralize one another. This leaves only the purely resistive component of the circuit to oppose the flow of current. Distractors often suggest that resistance becomes zero or equals reactance. However, resonance is strictly about the complete cancellation of the reactive components, regardless of the circuit's actual physical resistance.