This expression relates to the period of time that elapses prior to the establishment of a steady state. Initially the concentration of a reactive intermediate is zero, and it rises to the steady-state concentration during the transient phase.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 189 (https://doi.org/10.1351/pac199668010149)