This term is used to express the extra-column volume. Strictly speaking, the term 'dead-volume' refers to volumes within the chromatographic system which are not swept by the mobile phase. On the other hand, mobile phase is flowing through most of the extra-column volumes. Due to this ambiguity the use of term 'dead-volume' is discouraged.
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 833 (https://doi.org/10.1351/pac199365040819)
See also:
Orange Book, 2nd ed., p. 101 (https://media.iupac.org/publications/analytical_compendium/)