A measurement of electrical resistance in ohms (Ω) for AC electrical circuits such as in audio.
Bridging impedances, where a source’s output impedance is 10 times lower than the load’s input impedance, are specified for modern analog audio devices so that signal level (voltage) is properly transferred from one device to another, even if the signal is sent to multiple destinations
For headphones, low impedance phones (≤ 80 ohms) will play loudly with a low-power portable music player, like a phone. Higher impedance headphones (≥ 200 ohms) are more appropriate with high-power professional headphone amplifiers and studio equipment.
Passive loudspeakers are typically rated at 8Ω and should be powered by an amplifier that matches the speaker’s impedance.