What term describes the transport of molecules against their concentration gradient?

The correct term that describes the transport of molecules against their concentration gradient is active transport. This process requires energy, usually in the form of ATP, to move substances from an area of lower concentration to an area of higher concentration. Unlike passive transport, which relies on the natural movement of molecules and does not require energy, active transport is essential for maintaining concentration differences across cell membranes, allowing cells to intake necessary nutrients, ions, and to expel waste products efficiently. This mechanism is critical for various physiological processes, including nutrient absorption and ion regulation in cells.