Simple Definition:

A joule is a unit of measurement used to quantify energy. [Unit of measurement: a standard used to express a quantity] It is named after James Prescott Joule, a British physicist. The joule is used to measure various forms of energy such as mechanical, electrical, and thermal energy. [Mechanical energy: energy resulting from the movement or position of an object; Electrical energy: energy generated by the flow of electric charge; Thermal energy: the energy associated with the temperature of an object]

Examples:

• When you turn on a light bulb, the electrical energy consumed can be measured in joules.
• If you push a car up a hill, the energy used to perform that action can be measured in joules.

Very Simple Definition:

A joule is a way to measure energy. It tells us how much energy something has. For example, when you use electricity, the energy you use is measured in joules.