The opposition (measured in ohms) of circuit elements to an alternating current. The impedance includes both resistance and reactance. In an AC circuit, resistance presented to the current is a function of frequency, called impedance and is also measured in ohms. Ohm’s law can be used for AC if the reactances are zero, that is, if there are no capacitors or inductors in the circuit. See Appendix B.

source output impedance: the actual impedance of the output circuit. The output impedance of a device determines just how much current it can provide to a load.

source load impedance: the impedance that the source circuit is designed to serve as a load. This is usually the figure given in the specifications, e.g., the impedance value that the equipment was designed to be connected to.

load input impedance: the impedance that the load is designed to be fed from. As long as this value is higher than the source output impedance, the rest doesn’t matter. For example, if a tape recorder is connected to a mixer, this value is the impedance that the recorder wants to serve at its input.

load termination impedance: the actual impedance of the input circuit.