whats impedance?
Impedance is the correct term for what you are calling ohms.
It means the total opposition to current flow in an alternating current circuit and consists of both resistance and reactance where reactance comes from capacitive or inductive components. It is also measured in ohms and can sometimes be confusing as a result.
If you put an ohmmeter on your speakers you would only be measuring the DC resistance of the voice coil, not the impedance. Same thing with pickups - the resistance of the coil which is often quoted in advertisements is not the same as the impedance of a pickup.