Energy, in physics, is a property of objects which can be transferred to other objects or converted into different forms, but cannot be created or destroyed.
Energy can be expressed in negative units representing energy loss.
In SI units, energy is measured in joules, the energy transferred to an object by the mechanical work of moving it 1 metre against a force of 1 newton.
All of the many forms of energy are convertible to other kinds of energy, and obey the conservation of energy. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), elastic energy stored by stretching solid objects, chemical energy released when a fuel burns, and the thermal energy due to an object's temperature.
vCalc's Energy folder contains content that relates to the different forms of energy.