What is the term for an underground layer that holds water and allows it to flow through?

The term for an underground layer that holds water and allows it to flow through is an aquifer. Aquifers are geological formations, typically composed of rock or sediment, that have the capacity to store and transmit water. They are crucial for supplying groundwater, which can be accessed through wells and springs.

An aquifer must have both porosity and permeability; porosity refers to the storage capacity of the material, while permeability indicates the ability of water to flow through it. This makes aquifers essential for water supply in many regions, particularly where surface water is scarce.

Reservoirs, while also related to water storage, refer typically to surface water storage systems, often created by damming rivers. Streambeds, on the other hand, are channels where water flows above ground in rivers and streams, and do not store subsurface water as aquifers do. Permeable layers describe materials that allow water to pass through but do not necessarily indicate the presence of stored water or the characteristics of an aquifer.