The term 'battery unit' is composed of two words: 'volt' and 'CELL'. A volt is a unit of electrical potential difference, or the difference in energy between two points in an electric field. A CELL is a single unit of a battery, which is a device that stores energy and can be used to power electrical devices. Together, these two words form the term 'battery unit', which is a device that contains multiple cells and is used to store and provide electrical energy.