Understanding Impedance (Z)
Okay, let's start with a basic definition of impedance. We should first think about electrical resistance (represented by R), measured in Ohms (symbol Ω). Imagine a simple circuit consisting of a battery and a resistor. The battery generates a voltage which tries to force a current around the circuit connected between the battery's two terminals. The resistor resists that current — the higher the value of the resistor, the lower the current will be, and vice versa. In resisting the current, a voltage difference is developed across the resistor. This important phenomenon is defined mathematically in Ohm's Law, where the battery voltage (represented by V and measured in Volts) equals the current (represented by I and measured in Amps) multiplied by the resistor's resistance value. Expressing this law algebraically, V=IR, a simple bit of algebraic rearrangement gives I=V/R. So if the battery is 12V and the resistor is 120Ω, the current flowing around the circuit will be 12V/120Ω, which is 0.1A, or 100mA.
This simple example is of a Direct Current (DC) circuit — the battery voltage is steady and unchanging (ignoring the effect of the battery losing energy over time). However, when we are dealing with audio electronics, the signal voltage changes amplitude continuously to represent the changing amplitude of the audio signal, and it alternates between positive and negative cycles. The currents that flow therefore have varying amplitudes and alternate in direction as well, and we have what is known generically as an Alternating Current (AC) circuit.
This is where things become slightly more complex, because, in addition to the resistance, there are two other fundamental components which affect the current flowing around an AC circuit. In addition to the simple resistance we have already discussed, there is also capacitance and inductance to consider. In simplistic terms these also act like resistors, except that their resistance to current changes in proportion to the frequency of the signal voltage fluctuations — the rate at which the current flowing through the circuit is made to change direction by the audio signal voltage, in this case.
All audio electronics have combinations of resistors, capacitors and inductors connected in circuits, along with 'active' components like transistors or valves which provide amplification or act as switches. To make life slightly easier for ourselves, we often consider the total 'resistance' of a complex circuit involving resistors, capacitors and inductors as a composite lump, and that's what we call the impedance.
Impedance has the symbol Z — hence references to high-Z inputs, for example — and is still measured in Ohms. However, the actual value depends to some degree on the frequency of the signal voltages involved. In audio input and output circuits the impedances are principally resistive to make interconnection easier — the impedance won't change too much over the range of audio frequencies. However, the impedance to radio frequency (RF) signals will often be very different to that at audio frequencies in order to keep RF interference out.