Explanation: AC impedance, symbolized by Z, represents the total opposition a circuit presents to the flow of alternating current. It's a complex quantity that includes both resistance (R) and reactance (X), which arises from inductors and capacitors. Just as resistance is measured in Ohms, reactance (whether inductive or capacitive) is also measured in Ohms because it fundamentally opposes current flow. Since impedance is the vector sum of resistance and reactance, its unit of measurement is also the Ohm. This is a direct extension of Ohm's Law principles applied to AC circuits, crucial for understanding how radio circuits behave. Mho (or Siemens) is the unit for *conductance* or *admittance*, which are the reciprocals of resistance and impedance, respectively. Joule is the unit of *energy* or *work*, not the opposition to current flow.