Voltage is an alternative term for the electrical potential difference between two points. An electromotive force (EMF) is the voltage produced by a source of energy such as a battery or generator.
The SI unit for electrical potential difference is the volt. The unit is named after Italian physicist Alessandro Volta (1745 – 1827), so the unit symbol is given in upper case (e.g. spoken as "five volts" but written as 5 V).