Final answer:
An aquitard is a rock that is low in permeability, acting as a barrier to the flow of water.
Step-by-step explanation:
An aquitard is a rock that is low in permeability, so water does not readily pass through it. This means that an aquitard acts as a barrier to the flow of water. Unlike aquifers, which are rocks that can yield significant amounts of water, aquitards restrict the movement of water.
Examples of aquitards include shale and un-fractured igneous or metamorphic rock.