(1) Underground water. There are two types.
Open aquifers have permeable materials overlying them (like soil or loose gravel).
Closed aquifers are capped with an impervious layer like clay, which prevents water from infiltrating down from above.
The water level in aquifers rises and falls as water collects in them or drains away.
(2) A water-bearing rock formation or group of rock formations.
(3) Aquifers are water-permeable layers below the surface of the earth where water collects and is trapped by surrounding layers of rock that do not conduct water.
Aquifers provide water through springs and wells.
The depth that an aquifer begins at is the beginning of the water table and fluctuates with precipitation and use.
(1) I've never seen an aquifer, but I've walked all over them down there, in the ground somewhere.
(2) An aquifer is where ground water hangs out.
(3) The stream was dry, but the aquifer, lying close to the surface, was accessible by digging a shallow hole.