Electrode, a conductor through which an electric current enters or leaves a solution or other medium in an electrical device such as a battery, electrolyt ic cell, or electron tube. In some devices electrodes are also called poles or plates.

The electrodes of a battery are separated by a solution containing ions (electrically charged atoms or groups of atoms). One of the electrodes—the negative electrode—undergoes a chemical reaction that gives it excess electrons. The other electrode—the positive electrode—undergoes a chemical reaction that removes electrons. When the two electrodes are connected by an external electrical circuit, the excess electrons flow from the negative to the positive electrode.

Electrolytic cells and electron tubes are connected to an external source of electrical power, such as a battery or dynamo. The power source moves electrons into one of the electrodes (making it the negative electrode), withdraws electrons from the other (making it the positive electrode), and causes a current to flow through the medium between them.

The electrode through which negative charge enters an electrical device is called the cathode; the electrode through which negative charge leaves is called the anode. The negative electrode of a battery is thus the anode, whereas the negative electrode of an electrolytic cell is the cathode.