Unsaturated Zone: Zone in the upper layers of the soil, unconsolidated sediments, and bedrock where the pore spaces are not completely filled with water (i.e. not saturated). This is sometimes also known as the Vadose Zone.
Figure: Conceptual schematic of the vadose zone and groundwater. Source: ACSESS, Alliance of Crop, Soil, and Environmental Science Societies: https://dl.sciencesocieties.org/story/2016/sep/tue/small-scale-seismic-monitoring-of-varying-water-levels-in-granular-media-0