- 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.
- In <em>geology</em>, a water-bearing bed or stratum, necessarily of some open-textured rock.
- 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.
- An <xref>underground</xref> layer of water-bearing <xref>porous</xref> <xref>stone</xref>, <xref>earth</xref>, or <xref>gravel</xref>
- underground bed or layer yielding ground water for wells and springs etc