Seawater intrusion (SWI) arising from aquifer depletion is often classified as “active” or “passive”, depending on whether seawater moves in the same direction as groundwater flow or not. However, recent studies have demonstrated that alternative forms of active SWI show distinctly different characteristics, to the degree that the term “active SWI” may be misleading without additional qualification. In response, this article proposes to modify hydrogeology lexicon by defining and characterizing three classes of SWI, namely passive SWI, passive-active SWI and active SWI. The threshold parameter combinations for the onset of each form of SWI are developed using sharp-interface, steady-state analytical solutions. Numerical simulation is then applied to a hypothetical case study to test the developed theory and to provide additional insights into dispersive SWI behavior. The results indicate that the three classes of SWI are readily predictable, with the exception of active SWI occurring in the presence of distributed recharge. The key characteristics of each SWI class are described to distinguish their most defining features. For example, active SWI occurring in aquifers receiving distributed recharge only creates watertable salinization downstream of the groundwater mound and only where dispersion effects are significant. The revised classification of SWI proposed in this article, along with the analysis of thresholds and SWI characteristics, provides coastal aquifer custodians with an improved basis upon which to expect salinization mechanisms to impact freshwater availability following aquifer depletion.