Which quantity describes the voltage value at a specific moment on the AC waveform?

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Voltage on an AC waveform changes with time, so the value at a specific moment is the instantaneous voltage. This is what you read at that exact instant on a scope or fast voltmeter, and it’s typically written as E(inst) or v(t). The peak or maximum is simply the highest amplitude the waveform reaches, not the value at a particular moment. The RMS value is a time-averaged measure that’s used for power calculations, not a snapshot of the line at one instant. The average over a complete cycle is the mean value, which for a symmetric AC waveform is zero and doesn’t reflect the instantaneous reading either. So the instantaneous voltage is the quantity that describes the voltage at a specific moment on the waveform.