The capacity of a body or system to do work is called energy. In physics, energy is a measurable property associated with motion, position, heat, radiation, chemical reactions, and mass. Its SI unit is the joule (J), named after English physicist James Prescott Joule.
Energy appears in many forms. Kinetic energy belongs to moving objects, while potential energy is stored because of position or configuration—for example, a raised weight or a stretched spring. Chemical energy powers food and batteries; thermal energy relates to microscopic motion; and radiant energy is carried by electromagnetic waves such as sunlight. These forms can change into one another, but the total amount of energy is conserved.
A common mix-up is confusing energy with power or work. Work is the transfer of energy when a force moves an object, while power measures how quickly energy is transferred and is measured in watts. In advanced thermodynamics, not all of a system’s energy can necessarily be converted into useful work; concepts such as free energy and exergy describe the portion available under particular conditions. For a general physics definition, however, “energy” is the canonical answer.