A water bearing layer of rock within the ground.
Aquifers are the layers of sediments and rock that can hold and transmit water. They act like sponges; where water moves slowly through layers of gravel, sand, and hard rock under the ground.
Some aquifers are confined or separated from other groundwater or surface water systems by layers of impermeable rock or clay. Others are unconfined and may feed or collect water from springs, streams, and rivers above ground. Unconfined aquifers upper surface is known as the water table, like water encountered when you dig a hole in the sand at the beach.