Electrode potential is a measure of the tendency of a reaction at an electrode. An electrode of an element (M) placed in a solution of its ions (M+) constitutes a half-cell. In general a potential difference exists between the electrode and the solution, caused by reactions of the type M M+ + e. In practice it cannot be measured absolutely and standard electrode potentials are defined with reference to a hydrogen electrode under specified conditions of concentration, temperature, and pressure. Such reactions are oxidations, and so the term oxidation potential is often used. See also electrochemical series.