These definitions are written in general language to help learners understand these concepts. For technical definitions of terms, we recommend you consult the Soil Science Society of America (SSSA) Alliance of Crop, Soil, and Environmental Science Societies (ACSESS) glossary or the hydrogeology glossary in C.W. Fetter's Applied Hydrogeology (Fetter, C. W. [2018]. Applied hydrogeology. Long Grove, IL: Waveland Press).

Capillary action:
The capillary wicking or water flow that occurs due to capillary pressure.
Capillary pressure:
The pressure difference between the nonwetting fluid pressure minus the wetting fluid pressure.
Hydraulic conductivity:
Soil’s capacity to let water flow (or capability of the soil to transmit water). A low hydraulic conductivity means water has trouble moving within this soil.
Matric potential:
The potential of water to move from wet areas to dry areas within the pores of soil, derived from the force of individual particles holding on to water. If soil is too dry, soil particles will hold on to water and not let it flow: it would have low matric potential. Thus, matric potential is expressed as a negative quantity.
Pressure:
The pressure of and within water held within soil or rock. We consider measured water pressure relative to atmospheric pressure, which we designate zero pressure. Water pressure is equal to atmospheric pressure (or zero) at the water table. Water pressure increases to larger positive pressures with increasing distance below the water table. (It may help to think about scuba diving.) Similarly, water pressure becomes more negative (i.e., larger negative values or larger suction or vacuum) with increasing distance above the water table.
Pressure head:
The pore-water pressure in terms of length (pressure divided by the product of the water density and gravity), which is also equal to the height that water would rise at equilibrium above a location upon exposure to the atmosphere.
Pore spaces:
Non-solid spaces between particles of soil. (The same concept pertains to fractures in rock.)
Root zone:
The upper six inches of soil. Keep in mind that in the root zone, soil could include an additional 1-3% of organic matter. The water holding capacity of that soil would then increase, and these curves would look slightly different. Another factor that could change the particle size in the root zone is plowing, which can create a fluffy layer or a compacted layer.
Water saturation:
State of soil when all non-solid spaces are filled and the soil cannot hold any more water.
Soil water elevation:
Height between the water table and the surface of the soil.
Water content:
How much water by volume is in the soil relative to the total volume of the soil, where the total volume includes the solid and non-solid portions. Water content is related to hydraulic conductivity.
Water holding capacity:
The capacity of a given soil to hold on to water. These graphs show the water holding behavior of clay, sand, and loam soils, without organic matter. Keep in mind that in reality the 6-inch root zone at the surface could include 1-3% organic matter. Its water holding capacity would then increase, and these curves would look slightly different.
Wetting front:
the interface between newly wetted soil and the drier soil next to it.
Wilting point:
The threshold below which plants can no longer extract water from the soil. If there is less water than this in the soil, plants will wilt.