Impedance

The speaker’s electrical load in ohms — the amp-compatibility number.

Nominal impedance (8Ω, 6Ω, 4Ω) describes how hard a speaker is to drive. Lower impedance draws more current from the amplifier, and real impedance dips below nominal at certain frequencies. Most modern amps handle 6–8Ω speakers without drama; 4Ω towers deserve an amp rated for 4Ω loads. It is a compatibility check, not a quality score.