noun.
An underground layer of permeable rock, sediment, or soil that yields water. Aquifers can range from a few square kilometers to thousands of square kilometers in size.
The Century Dictionary and Cyclopedia
noun.
In geology, a water-bearing bed or stratum, necessarily of some open-textured rock.
the GNU version of the Collaborative International Dictionary of English
noun.
an underground bed or layer yielding ground water for wells and springs etc.. The water contained in an aquifer may be of great age, and in such cases is sometimes called fossil water.