Electromotive force, also called emf (denoted and measured in volts), is the voltage developed by any source of electrical energy such as a battery or dynamo.
The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts.
In electromagnetic induction, emf can be defined around a closed loop as the electromagnetic work that would be transferred to a unit ofcharge if it travels once around that loop. (While the charge travels around the loop, it can simultaneously lose the energy via resistance into thermal energy.) For a time-varying magnetic flux impinging a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop.
In a two-terminal device (such as an electrochemical cell or electromagnetic generator), the emf can be measured as voltage across the two open-circuited terminals. The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.