What is the nominal supply voltage?

The term, 'nominal', simply means 'named'. For example, the nominal supply voltage for a residence in the UK is 230 V. But this doesn't mean that the actual supply voltage is 230 V, because the supply voltage is allowed to vary by -6% and +10% of the nominal voltage. In other words, the actual voltage could be anywhere within the range of 216.2 V to 253 V. In fact, during periods of heavy load, the actual voltage would tend towards the lower end of this range, whereas during periods of higher load, the actual voltage would vary towards the upper end.