What is resonance in RLC circuits and what are its characteristics?

Resonance occurs when inductive and capacitive reactances are equal (XL = XC), causing them to cancel out. At resonant frequency fr = 1/(2*pi*sqrt(LC)), the circuit impedance is purely resistive and minimum (series RLC) or maximum (parallel RLC). Characteristics include maximum current in series circuits, minimum current in parallel circuits, unity power factor, and high Q-factor circuits exhibiting sharp frequency selectivity.