The term for the movement of substances across a cell membrane against a concentration gradient is known as active transport. This process requires energy, usually derived from ATP, to move molecules from an area of lower concentration to one of higher concentration. Active transport is crucial for maintaining the necessary concentrations of ions and nutrients inside cells, enabling them to perform essential functions. Examples of active transport mechanisms include the sodium-potassium pump and the transport of glucose in intestinal cells. This process ensures that cells can maintain homeostasis and function efficiently.