An aquifer is a geological formation that can store, transmit, and yield water to wells or springs. Aquifers are composed of permeable materials such as sand, gravel, or fractured rock. They can be confined, with impermeable layers above and below, or unconfined, where they are directly recharged by surface water. Aquifers are crucial for water supply in many regions and are a primary focus in the study of hydrogeology.
Reference: Freeze, R. A., & Cherry, J. A. (1979). Groundwater. Prentice-Hall.