Impedance (Z) is the calculated value of Inductance (L), Capacitance (C), and Resistance (R).
There are two specifications for Impedance:
Nominal which is more or less an average value, and minimum, which is a critical value to know when matching loudspeakers to amplifiers. Typically, the lower the frequency the lower the applied impedance the loudspeaker presents to the amplifier. Under normal conditions a speaker’s impedance may fluctuate from 3.2 Ohm Z to as high as 17- or 18-Ohm Z.
Rule of Thumb:
A 4Ω amplifier can drive an 8Ω loudspeaker but it will produce 50% less power. Conversely an 8Ω amp driving a 4Ω loudspeaker produces 50% more power.
You CANNOT measure the Impedance of a loudspeaker with a DC Ohmmeter and get an accurate reading. All you can check for is open or short circuit.