A @[email protected] that corresponds to the reduction or @[email protected] of an @[email protected] formed by a prior @[email protected] from another substance that is not electroactive, and that is partially or entirely controlled by the rate of that reaction. The reaction may be heterogeneous, occurring at an electrode-solution @[email protected] (surface reaction), or it may be homogeneous, occurring at some distance from the @[email protected] (volume reaction).