A joule (J) is the unit of energy used in the International System of Units (SI). It is named after English physicist James Prescott Joule, who developed the theory of conservation of energy and the first law of thermodynamics in the 19th century.

Following SI conventions, joule is abbreviated with a capital J because the term is derived from a proper name, though if spelled out, it is not capitalized. A joule is defined as the amount of energy expended by a force of one newton moving an object one meter in the same direction as the force.

The above definition of a joule may sound complicated, but the great thing about SI units is that they can all be defined in terms of grams, meters, and seconds, familiar concepts to most people. A joule can therefore be defined by the equation J = kg (m²/s²). While James Prescott Joule first discovered the value of the joule through his work on heat and mechanical work, Dr. Mayer of Heilbronn introduced the term.

The term newton-meter (N.m) is interchangeable with joule. Joule can be used to measure not only a force in the same direction of movement, or work, but also a force perpendicular to the direction of movement, or torque. A watt, the SI unit of power, follows from the joule; it is defined as one joule per second. In addition to being equivalent to a newton-meter, a joule is equal to a watt-second (W.s). A kilowatt-hour (kW.h), often seen on household electricity meters, is equal to 3600 J.

To get an idea of the joule in everyday terms, consider the following examples. A joule is the amount of energy needed to raise the temperature of cool, dry air by one degree Celsius. A quiet person produces 100 joules of heat energy every second. Finally, the amount of energy required to lift a small apple one meter against the Earth's gravity is roughly equivalent to one joule.