The voltage developed by a battery or a dynamo or any other electrical source of energy is defined as Electromotive force or EMFs. It is denoted by (E), and its measuring unit is (V) volts.
Example of such devices that produces EMFs are electrochemical cells, solar cells, transformers and electric generators, photodiode, etc., by which electric potential is generated. Here the word FORCE means energy per unit charge.
Taking an example of electrochemical cell.
Take two electrodes one of copper (Cu) which is dipped in copper sulphate solution and another of zinc (Zn) which is dipped in zinc sulphate solution and the two rods or electrodes are connected by a galvanometer which shows the deflection indicating the electric potential or the difference between the two voltages.