A specification of the load a loudspeaker places on an amplifier, measured in ohms. It tends to oversimplify the actual situation because the impedance of a loudspeaker varies according to frequency. So a speaker with a nominal impedance of eight ohms may actually present a load of six ohms at some frequencies and thirty ohms at others. Common loudspeaker impedances are four, six and eight ohms. Back in the 1960s, higher impedance speakers -- 16 and 32 ohms -- were fairly common.