This letter proposes a measurement-based definition for the phase-angle of the negative-sequence voltage that fits in the general approach used in the IEC standard for power-quality measurements, IEC 61000-4-30. The definition is based on an expression that is obtained during a derivation of the classical expression for calculating the negative-sequence voltage from the rms values of the three line-to-line voltages. The definition can be applied to the 10/12 cycle, 150/180-cycle and 10-minute values.