electromotive force
/ ih-lek-troh-MOH-tiv force /
Think of a battery as a pump for electric charge, the way a water pump lifts water uphill. Electromotive force, or emf, is the strength of that push — the total voltage a cell can muster to drive electrons around a circuit when nothing is dragging on it. Despite the word "force," it is not a force at all; it is measured in volts, an energy-per-charge.
More precisely, the emf of a cell is the difference between the electrode potentials of its two halves, measured when no current is flowing — that is, the maximum voltage the cell could ever supply. It comes from the two half-reactions: subtract the smaller electrode potential from the larger, and the result is the cell's emf. A positive emf signals a spontaneous reaction, one that can drive a current on its own.
Emf matters because it sets a cell's headroom. The bigger the emf, the harder the cell can push current and the more electrical work it can do per electron. But the moment current actually flows, the voltage you measure drops a little below the emf, because some push is wasted overcoming the cell's own internal resistance — which is why a battery under heavy load reads lower than its rating.
A single AA battery is rated at 1.5 volts — that is its emf. Hold a voltmeter across it with nothing connected and you read about 1.5 V; run it through a motor and the working voltage sags a little as current flows.
Emf is the open-circuit voltage; under load the working voltage drops a bit.
Emf and "terminal voltage" are not the same. Emf is the ideal, no-current push; terminal voltage is what you actually get at the posts once internal resistance has taken its cut.