What is the measure of the amount of water held by a rock or soil in pores or voids, expressed as a percentage of total volume?

The measure of the amount of water held by a rock or soil in pores or voids, expressed as a percentage of total volume, is known as porosity. Porosity indicates how much of a material's total volume consists of empty spaces where water can be stored. It is a critical property in fields like hydrogeology and environmental science, as it influences water movement and availability in soils and rock formations.

In contrast, permeability refers to the ability of a material to transmit water through its pores, while specific retention measures the amount of water retained in the pores against gravitational forces. Specific gravity, on the other hand, is a comparative density measure of a material relative to water. Understanding porosity helps in assessing groundwater resources, soil behavior in engineering, and various aspects of environmental health.