The current that flows, at any particular value of the applied potential, in the absence of the substance whose behaviour is being investigated (i.e. in a blank solution).
Source:
PAC, 1985, 57, 1491. 'Recommended terms, symbols, and definitions for electroanalytical chemistry (Recommendations 1985)' on page 1499 (https://doi.org/10.1351/pac198557101491)