Final answer:
An aquitard is a unit of earth material that is porous but has low permeability, meaning it does not transmit groundwater freely. Examples of aquitards include clay layers and unfractured igneous or metamorphic rocks.
Step-by-step explanation:
A unit of earth material that may be porous but has low permeability so may store water but does not transmit groundwater freely is called an aquitard. Aquitards have low permeability, meaning they restrict the flow of groundwater. Examples of aquitards include clay layers and unfractured igneous or metamorphic rocks.