Impedance is the opposition to flow of alternating current, like that generated by the bioelectrical neural activity of the brain.
It has two components, resistance and reactance. With regards to electrode impedance, the main interface between equipment (i.e. electrode) and human is the skins surface. The voltage fluctuations that we measure are transmitted (or one might say conducted) through the fluid of the body. The current can also relatively easily flow through the wires and circuitry of the instruments that we use. However, the interface is where the challenge lies – the surface of the skin. When we place the electrode on the surface of the skin, the current flow is impeded by dead skin cells and other impurities.
In order to overcome this, what we do is scrub the skin clean and use conducting gel which acts as a capacitor/resister…and we ‘lower’ the impedance i.e. we bridge the interface between fluid filled tissues of the body, and the instruments.
Impedance is usually measured by passing a known, low-level current through the circuit (including the interface with the skin) and measuring the opposition to the flow of this current.