The movement of an ion down its concentration gradient is called diffusion. More specifically, when ions move from an area of higher concentration to an area of lower concentration through a membrane, this process is passive and does not require energy. This movement occurs until equilibrium is reached

. This type of movement is often described as moving "down the concentration gradient" and is driven by the difference in ion concentration across the membrane. It is a form of passive transport because it relies on the natural kinetic energy of ions and does not require cellular energy input

. If the ion movement also involves the electrical charge difference across the membrane, the combined driving force is called the electrochemical gradient. But the basic movement down the concentration gradient alone is diffusion

. In summary:

• Movement of an ion down its concentration gradient = diffusion (a passive process)

• When considering both concentration and electrical gradients, the driving force is the electrochemical gradient

If ions move against their concentration gradient (from low to high concentration), that process is called active transport , which requires energy

. But your question specifically asks about movement down the concentration gradient, which is diffusion.