Nominal literally means 'as named', so it is whatever the manufacturer or whoever says it is.
With speakers, the typical amplifier isn't bothered by high impedance within reasonable bounds, but can be overloaded by low impedance. Since speakers typically have these high and low impedance peaks, the nominal impedance is generally called a little higher than the low points, and then usually in powers of 2--2/4/8/16 ohms. That's just a tradition and really only applies to normal hi-fi speakers, not PA or other types. There's no reason you couldn't have a 6 ohm or 10 ohm nominal rated speaker.
I would call this one an 8 ohm speaker and I don't think too many would disagree.