Time required for a liquid in a capillary to move due to electro-osmosis through the effective length of the capillary, Leff. This time is usually measured as the migration time of a neutral compound, called an electro-osmotic flow marker which is assumed to have an electro-osmotic mobility that is negligible compared to that of the analyte.
Source:
PAC, 2004, 76, 443. 'Terminology for analytical capillary electromigration techniques (IUPAC Recommendations 2003)' on page 447 (https://doi.org/10.1351/pac200476020443)