Underground surface below which the ground is wholly saturated with water WordNet 3.0 & Open English WordNet

The water table is the upper surface of the zone of saturation. Wikipedia

Water that occupies pores, cavities, cracks and other spaces in the crustal rocks. It includes water precipitated from the atmosphere which has percolated through the soil, water that has risen from deep magmatic sources liberated during igneous activity and fossil water retained in sedimentary rocks since their formation. The presence of groundwater is necessary for virtually all weathering processes to operate. Phreatic water is synonymous with groundwater and is the most important source of any water supply. OmegaWiki

The level, at a particular site, below which the subsoil and rock masses of the earth are fully saturated with water. Wiktionary

The level of the water table below ground. Wiktionary

Level of water table below ground. Wiktionary (translation)

The level, underground, below which the ground is saturated with water. Wiktionary

Level underground. Wiktionary (translation)