AQUIFER (EN: waterbearing horizon, water-bearing level; DE: wasserfuhrender Horizont, Wasserfuhrer, Wasserwog, Wassersohle; FR: nappe aquifere; ES: nivel acuifero; RU: водоносный горизонт) are the layers of the water-permeable rocks, which (layers) are homogeneous or similar according to the facial-lithological composition and hydrogelogical properties, the voids (pores, cracks) of which (of the rocks) are filled with the gravitational waters. According to the conditions of the embedment, they distinguish the following aquifers: within the regions of the permafrost, namely, above-permafrost, inter-permafrost, and below-permafrost; within the petroleum-and-gas-bearing regions, namely, contoured, upper, lower, and intermediate; within the regions of the ore and coal deposits, namely, above-ore (above-coal), and below-ore (below-coal); within the salt deposits, namely, above-salt, inter-salt, below-salt, and near-salt. According to the hydraulic conditions, they distinguish the pressurized (between the water-resistant strata) aquifers, and the non-pressurized (or ground) aquifers; according to the internal structure, they distinguish the single-layer, double-layer, and multi-layer aquifers; according to the type of the water-resistant bed, they distinguish the inclined, horizontal, and mixed aquifers.

Within the mining craft, the aquifer significantly complicates the conducting of the preparatory mine workings, and of the extractive works. For the elimination of the negative impact of the aquifer, there is performed the control over its regime and resources, using the methods for the decreasing of the level of the water, plugging, freezing, electric drying, and so on. For the isolation of the aquifer, there is also prospective the usage of the barrage.