Final answer:
The movement of an ion down its electrochemical gradient is called passive transport, a process that does not require energy from the cell.
Step-by-step explanation:
The movement of an ion down its electrochemical gradient is known as passive transport. This process does not require energy input from the cell. Ions move across the cell membrane through passive transport mechanisms like diffusion, osmosis, and facilitated diffusion. In passive transport, substances move from an area of higher concentration to an area of lower concentration without the utilization of energy, following their electrochemical and concentration gradients.