Impedance
Impedance is the total opposition a circuit or component presents to alternating current, measured in ohms. It includes ordinary resistance plus reactance from inductance and capacitance, so it can change with frequency. Amplifiers and speakers are designed to work with a specific impedance, such as 4, 8 or 16 ohms.
Example: A guitar speaker labeled 8 ohms has an impedance that is roughly 8 ohms across most of its range but rises at higher frequencies because of the inductance of its voice coil.