1. The three-dimensional sub-surface geometry of a geologic rock formation (or, a group of rock formations or part of a formation) that contains groundwater in the spaces between sediment grains, voids, or fractures.
  2. A geological formation or structure that can store and/or transmit water to wells and springs. The use of the term aquifer is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply source.
    See also Confined Aquifer, Unconfined Aquifer.