The voltage between any two specified points of an electrical conductor, such as the terminals of a circuit element or component, due to the flow of current between them.

In the case of direct current the voltage drop is equal to the product of the current in amperes and the resistance in ohms. In the case of alternating current the product of the current in amperes and the resistance in ohms is the resistance drop, which is in phase with the current. The product of the current and the reactance gives the reactance drop, which is in quadrature with the current.