What is the power frequency voltage?

It's unclear whether you are asking about frequency or voltage.

The frequency and voltage are specified in the relevant national standards, regulations, and legislation.

In North America, for example, the residential nominal voltage is 240/120 V at 60 Hz. In Europe, the residential nominal voltage and frequency is 230 V at 50 Hz.

By 'nominal', we mean 'named'. National regulations allow these values to vary within specified limits.