Define the term 'impedance'.

Impedance is defined as the total opposition to the flow of alternating current (AC) in a circuit, which encompasses both resistance and reactance. Resistance refers to the opposition to current flow due to the material properties, while reactance accounts for the opposition due to capacitors and inductors in the circuit, which can store and release energy.

In AC circuits, the effects of reactance become significant due to the time-varying nature of the voltage and current, leading to phase differences between them. Impedance is not only a measure of how much the circuit resists the flow of current but also how it responds to changes in frequency. It is typically represented in complex form, combining both resistive and reactive elements.

The other options focus on specific aspects of electrical behavior: the first choice pertains solely to direct current and does not incorporate the broader context of AC; the third choice describes a different concept of energy storage rather than opposition to current; the fourth choice concerns time delay, which is related to phase shifts rather than opposition to current flow. Thus, the most comprehensive and accurate definition relevant to AC is that of impedance as the total opposition to its flow.