Impedance of headphones

The total opposition that an electrical circuit (in this case, a headphone speaker) presents to an alternating current (audio signals are alternating currents, too).
The impedance is important in determining whether it is possible to connect several pairs of headphones in parallel to a single audio source. This will only work if the headphones have a relatively high impedance, e.g., 600 ohms as the AKG K 240 Monitor.
Headphones for use with audio sources with a low output voltage, i.e., most portable equipment, should have a low impedance, e.g., 55 ohms.