Final answer:
An aquitard is a rock or sediment with water-filled pores wherein upward percolation is blocked by an overlying impermeable layer. It acts as a confining layer and prevents the flow of groundwater.
Step-by-step explanation:
A rock or sediment with water-filled pores wherein upward percolation is blocked by an overlying impermeable layer is termed an Aquitard. Aquitards act as confining layers and prevent the flow of groundwater. They are made up of impermeable rocks such as shale, or un-fractured igneous or metamorphic rock. Aquitards are different from aquifers, which are permeable rocks that can yield significant amounts of water.