Half the peak-to-peak amplitude of a sinusoidal alternating current.
Sources:
Orange Book, 2nd ed., p. 53 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1985, 57, 1491. 'Recommended terms, symbols, and definitions for electroanalytical chemistry (Recommendations 1985)' on page 1494 (https://doi.org/10.1351/pac198557101491)