noun
1. : a molecular mechanism by which sodium ions are actively transported across a cell membrane ; especially : one by which a high concentration of potassium ions and a low concentration of sodium ions are maintained within a cell and that is controlled by a specialized plasma membrane protein linking the hydrolysis of ATP to the active transport of intracellular sodium ions out of the cell and extracellular potassium ions into the cell to create an electrical and chemical gradient across the plasma membrane necessary for vital cell functions (as nerve cell excitation and cell volume regulation)
2. : the specialized plasma membrane protein that controls the sodium pump