There're a lot of definitions about energy. Generally," Energy" is the ability to do work. Work meaning something like lifting a mass, compressing a gas, producing heat, sound, light, etc.
"Energy is a capacity to do work".Energy exists in several forms such as potential energy,kinetic energy,heat,light energy,mechanical energy and many other forms....
In physics, energy is one of the basic quantitative properties describing a physical system or object's state. Energy can be transformed (converted) among a number of forms that may each manifest and be measurable in differing ways. The law of conservation of energy states that the (total) energy of a system can increase or decrease only by transferring it in or out of the system. The total energy of a system can be calculated by simple addition when it is composed of multiple non-interacting parts or has multiple distinct forms of energy. Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, and various types of potential energy such as gravitational andelastic. Energy is measured in SI units of joules (J). Common types of energy transfer and transformation include processes such as heating a material, performing mechanical work on an object, generating or making use of electric energy, and many chemical reactions.Units of measurement for energy are usually defined via a work process. The work performed by a given body on another is defined in physics as theforce (SI unit: newton) applied by the given body, multiplied by the distance (SI unit: metre) of movement against the opposing force exerted by the other body. Thus, the energy unit is the newton-metre, which is called the joule. The SI unit of power (energy per unit time) is the watt, which is simply a joule per second. Thus, a joule is a watt-second, so 3600 joules equal a watt-hour. The CGS energy unit is the erg, and the imperial and US customary unit is the foot pound. Other energy units such as the electron volt, food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of science and commerce and have unit conversion factors relating them to the joule.