Final answer:
The measure of open space in materials is called porosity, which, along with the connectivity of the spaces, determines the permeability of the material for fluid transport, affecting its ability to act as an aquifer or a confining layer.
Step-by-step explanation:
The measure of open space in materials such as rock, sand, and gravel is known as porosity. Porosity is a critical factor that influences the permeability of geological materials, which is the ability to transmit fluids. Although a material may be porous, it is the connectivity between the spaces that determines the material's permeability. For example, if the open spaces in a porous substance like sandstone are well connected, it can act as an aquifer, allowing water to flow through it efficiently. Sedimentary rocks such as limestone and sandstone often make good aquifers, while rocks like shale, or un-fractured igneous and metamorphic rocks, tend to be impermeable and act as confining layers, inhibiting the flow of water.