1. An underground bed or layer of earth, gravel, or porous stone that yields water.
2. Permeable underground rock stratum which holds water.
3. Any geological formation containing or conducting ground water; especially, one that supplies the water for wells, springs, etc.

The pore spaces in aquifers are filled with water and are interconnected, so that water flows through them. Sandstones, unconsolidated gravels, and porous limestones make the best aquifers. They can range from a few square kilometers to thousands of square kilometers in size.

An underground water supply which comes from porous rock, sand, gravel, etc.: "The alluvial aquifer provided the community with ground water that made it possible for their wells and springs to exist."