Incidentally I have never seen a speaker whose impedence measures a flat 2.5-3 ohms over 100-10k Hz. Have you?
I have the Thiel 3.6's. From Stereophile:
Impedance is a combination of linear R (resistance) and non-linear X (reactance)
A Reactance is a combination of Inductive and capacitive components. In the world of complex algebra the impedance Z is represented by = R + jX. In case of the DC, X component is null so the Impedance in this case is linear resistance R.
Inductive component impedance is known as wL where w is frequency and L is Inductance.
Capacitive component impedance is known as 1/wC where C is capacitance
Overall Z = R + j(1/wC + wL) where j is directional vector
Notably, with increase of frequency Inductive component increases while capacitive component decreases. that means that on DC capacitor will have an infinite impedance and Inductor will act like short circuit.
I think nominal impedance means the industry standard. Impedance should not fluctuate more then 8 or 9% from the standard. I.E. An 8Ohm speaker should be no less then 6Ohms, a 4Ohm speaker should be no less then 3Ohms, if they follow the standard.