Define an aquifer.

An aquifer is defined as a geological formation that holds groundwater. This concept is crucial in understanding how water is stored and accessed in the environment. Aquifers are typically composed of permeable materials, such as sand, gravel, or porous rock, which allow water to flow through them. This stored groundwater can be tapped for various uses, such as irrigation, drinking water supply, and industrial processes.

The significance of aquifers lies in their role in the hydrologic cycle and their contribution to the availability of fresh water. They act as natural reservoirs, filtering and storing water beneath the Earth's surface. This makes understanding and protecting aquifers essential for sustainable water management and ensuring access to clean water for human consumption and ecological health.