1. A permeable underground bed or layer of earth, gravel, or porous stone that holds and yields water: The farmer was fortunate to have an aquifer on his land so he drilled into it carefully in order to install a well.
2. Any geological formation containing or conducting ground water; especially, one that supplies the water for wells, springs, etc.: Alfred and the other geology students from the university prepared a careful study of the aquifers in their geographical region.

The spaces in aquifers are filled with water and consist of sandstones, gravels, and porous limestones that are interconnected, so that water flows through them.

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 continue functioning.