What term describes the combination of resistance and reactance in an AC circuit?

The term that describes the combination of resistance and reactance in an AC circuit is impedance. Impedance is a complex quantity that embodies both the opposition to current flow due to resistance (the real part) and the opposition to changes in current due to reactance (the imaginary part, which can result from inductors and capacitors).

Reactance itself can be broken down into inductive reactance and capacitive reactance. While resistance is related strictly to the dissipative aspect of a circuit where energy is converted into heat, reactance pertains to the energy storage properties of the circuit components. Impedance, therefore, provides a comprehensive measure of both these effects, allowing for a full analysis of AC circuits. This is particularly useful when analyzing phase relationships and total current flow in these systems.

Understanding impedance is crucial for analyzing electrical circuits that operate at varying frequencies, which are common in AC systems.