Final answer:
Active transport is the process wherein ions are moved across a cell membrane against the concentration gradient using energy, typically in the form of ATP.
Step-by-step explanation:
The process that moves an ion across a cell membrane against the concentration gradient is known as active transport. In contrast to facilitated diffusion and passive transport, which do not require energy and move substances down a concentration gradient, active transport requires energy. This energy is typically in the form of ATP (adenosine triphosphate). Within the cell membrane, specific proteins act as pumps, using this energy to move ions or molecules from an area of lower concentration to a higher concentration, effectively moving them 'uphill' against the gradient. An example of active transport is the sodium-potassium pump, which transports sodium out of the cell and potassium into the cell against their respective concentration gradients.