A property of matter that has the ability to make something happen through movement or a change in condition: These changes can be physical or chemical, and allow energy to be converted to another form, for example the chemical energy of fuel is converted into heat and then into mechanical energy in an engine.

The impetus to do work; different forms of energy can be converted to other forms, but the total amount of energy remains the same: Energy has several forms, some of which are easily convertible and can be changed to another form useful for work.

Most of the world's convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks.

Electrical energy is usually measured in kilowatt hours, while heat energy is usually measured in British thermal units (Btu).

The energy or power needed to start a response: For each type of reaction, activation energy is the minimum amount of collision energy that will drive reactant molecules to an activated condition, from which the reaction will proceed spontaneously.

A general term for renewable energy produced from biomass: Biomass energy can be produced by using wood and wood wastes, agricultural crops and refuse, or municipal and industrial debris.

The ability of an electric current to produce work, heat, light, or other forms of energy: Electric energy is measured in kilowatt hours.

The energy that a moving body possesses because of its motion: The kinetic energy of an object is dependent on its mass and the rate at which it is moving; equal to 1/2 mv², where m is mass and v is velocity.

When something moves, it is said to have kinetic energy.

To make a moving car stop, all its kinetic energy must be converted into other forms by heating up the brakes, for example.

Most physical systems contain stored, or potential, energy that can be turned into other kinds of useful energy at a later time: A car at the top of a hill has potential energy because of its position. It can turn its potential energy into kinetic energy by going down the slope.