The term that describes the decrease in the concentration of ions per unit volume when mixing an acid or base with water is “dilution.” Dilution involves adding a solvent, usually water, to a concentrated solution, resulting in a decrease in the concentration of solute particles (ions) per unit volume. This process does not alter the total number of ions present but disperses them in a larger volume, reducing their concentration. Dilution is a common technique used to control the strength of acids and bases for various applications, allowing for safer handling and precise adjustments of solution concentrations.