Final answer:
A layer of rock or sediment with exceptionally low permeability is termed an aquitard in hydrogeologic contexts.
Step-by-step explanation:
A layer of rock or sediment with exceptionally low permeability is termed a(n) aquitard in hydrogeologic contexts. Permeability refers to the ability of geologic material to transport fluids, and rocks that are impermeable make confining layers and prevent the flow of water. Examples of confining layers would be sedimentary rocks like shale or un-fractured igneous or metamorphic rock.