Reactance

Reactance X is the imaginary part of impedance, measured in ohms, characterising how strongly a reactive element (inductor or capacitor) opposes AC current flow at a given frequency. Inductive reactance X_L = ωL grows linearly with frequency — inductors are open circuits at high frequency and short circuits at DC. Capacitive reactance X_C = 1/(ωC) in magnitude (with a negative sign by convention, reflecting the −j in capacitor impedance) falls inversely with frequency — capacitors are open circuits at DC and short circuits at high frequency.

Unlike resistance, reactance does not dissipate power. A pure inductor or capacitor cycles energy in and out of its field each period: during one quarter-cycle it absorbs energy from the source and stores it (½LI² or ½CV²), during the next quarter-cycle it returns it. The integrated power over a full cycle is zero. This is why reactive elements are "loss-free" in principle (actual inductors and capacitors have small resistive losses; ideal ones don't).

The sign convention matters. An RL circuit has X > 0 (current lags voltage), an RC circuit has X < 0 (current leads voltage), and an RLC circuit at resonance has X = 0 (current and voltage in phase, impedance purely resistive, maximum power transfer at matched R). Reactance is why filter circuits work: a low-pass RC filter is just a voltage divider where the "high" arm is R (constant) and the "low" arm is |X_C| = 1/(ωC) (falls with frequency), so high frequencies get attenuated. Every audio equaliser, every radio IF filter, every DSL line-conditioner is built from combinations of reactances.