Final answer:
Porosity is the percentage of a material's total volume made up by empty spaces or voids, affecting how fluids move through substrates like soil and rock.
Step-by-step explanation:
Porosity is the percentage total volume of rock, sediments, or soil that is taken up by voids, which are essentially the empty spaces between particles in a material. In the geological context, porosity can greatly affect how water and other fluids move through underground materials. For example, sedimentary rocks which are composed of well-sorted sediment generally have a higher porosity due to the larger spaces between the individual grains. On the other hand, rocks like igneous and metamorphic with tight interlocking crystals have lower porosity. It is crucial in fields such as geosciences and engineering to understand the porosity of materials to predict the movement and availability of groundwater, as well as the stability of structures built on such materials.