1. A geologic formation, a group of formations, or a part of a formation that is water bearing.
  2. A geological formation or structure that stores or transmits water, or both, such as to wells and springs.
  3. An underground layer of porous rock, sand, or gravel containing large amounts of water. Use of the term is usually restricted to those water-bearing structures capable of yielding water in sufficient quantity to constitute a usable supply.
  4. A sand, gravel, or rock formation capable of storing or conveying water below the surface of the land.
  5. A geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.

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