Water retention is the property of a substance, material, or system to hold or store water, often against the influence of gravitational or other external forces. As a fundamental academic concept and area of research, it investigates the physical, chemical, and biological mechanisms governing this capacity, quantifies the amount of water held and the energy status of this water (e.g., water potential), and analyzes the intrinsic properties and environmental conditions that influence water storage, holding critical significance across disciplines such as soil science, hydrology, ecology, and materials science.