Marine life thrives throughout the ocean, with species abundance often greatest below the surface—making marine extremes across the water column critical to ecosystem health. Traditionally, marine heatwaves (MHWs) have been characterized using surface data, yet subsurface processes—such as nutrient recycling, (de)oxygenation, and carbon transport—play essential roles. Consequently, there is growing focus on subsurface MHWs, which are prolonged warm temperature extremes exhibiting diverse vertical structures and driving mechanisms. These events may be confined to the surface mixed layer, extend through the entire water column, occur exclusively below the surface without a surface signature, or even be isolated near the seafloor. Drawing on existing literature and new analyses, we propose a comprehensive naming convention that distinguishes between mixed layer, deep, thermocline, submerged, and benthic MHWs. While surface MHWs are linked to heat fluxes and shallow ocean dynamics, many subsurface events arise from vertical or horizontal displacement of temperature gradients, deep advection, or subduction of warm waters. Given sparse observations and the challenges in identifying these events, our proposed nomenclature aims to unify interdisciplinary studies and improve understanding of the biogeochemical impacts of subsurface MHW extremes.