The algebraic sum of the charges present at the surface of the particle divided by the elementary charge of the proton. The symbol z is also used for charge number of an ion.
Source:
PAC, 1994, 66, 891. 'Quantities and units for electrophoresis in the clinical laboratory (IUPAC Recommendations 1994)' on page 895 (https://doi.org/10.1351/pac199466040891)