For some years the author has used the term “infiltration” to describe the process involved where water soaks into or is absorbed by the soil. Absorption, imbibition, and percolation are often used in much the same sense. It seems better to confine the use of “percolation” to the free downward flow by gravity of water in the zone of aeration—a process for which a distinctive term is needed. “Absorption” includes the entrance of air as well as water, both liquid and vapor, into the soil (see Patten and Gallagher, Absorption of vapors and gases by soils, U.S. Dep. Agric., Bul. 51, Bur. Soils, Wash., 1908; also Charles H. Lee, On absorption and transpiration, Trans. Amer. Geophys. Union, 1932, pp. 288-298). “Infiltration” is limited to water in the liquid form and is more accurately descriptive of the physical processes by which rain enters the soil. “Water-penetration” is also sometimes used as if synonymous with infiltration. Its use should be restricted to the depth below soil-surface reached by the given surface infiltration.