Refers to the concentration gradient of an ion or molecule. The concentration gradient may exist across a biological membrane, where the concentration is higher on one side of the membrane compared to the other side. Concentration gradient may also exist in a solution without an apparent barrier separating the area of higher concentration from the area of lower concentration. In both cases, the free energy that results from the concentration difference drives the movement of the ion/molcule from the area of higher concentration to the area of lower concentration. In free solution, the ion/molecule simply diffuses down its gradient. Movement across a biological membrane is more complicated and is a function of lipid solubility of the ion/molecule as well as the presence of channels or transport proteins that can allow the ion/molecule to cross the membrane (see Lipid Bilayer Permeability and Summary of Membrane Transport Processes).
The flow of charge. In electrical wires and electronic circuits, current is carried by electrons. In physiological solutions, current is carried by ions in solutions.
The rate of movement of a substance across an interface. The interface could be the plasma membrane (separating the intracellular or extracellular fluid compartments), an epithelial sheet separating two compartments, or where two solutions of different composition meet.
Flux is reported as a rate. It is the amount of substance that moves across a given interface per unit time.