Definition: Energy is the capacity of a physical system to perform
work. Energy exists in several forms such as
heat,
kinetic or mechanical energy, light,
potential energy, electrical, or other forms.
According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form. Two billiard balls colliding, for example, may come to rest, with the resulting energy becoming sound and perhaps a bit of heat at the point of collision.
The SI unit of energy is the joule (J) or newton-meter (N * m). The joule is also the SI unit of work.
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