Energy is a term often used but rarely defined. It appears to have many definitions because it is used in such a broad variety of contexts. The word energy comes from the greek word energos that means "active, working". Lets take a minute to address the different situations where we here the term energy used:
1. In science class: When the word energy is used in science class, we may be talking about kinetic, gravitational, electrical, magnetic, mechanical, heat, nuclear, light, or chemical energy. In addition to these types there is also potential energy, which is the untapped energy that could "potentially" do work. Energy in a scientific concept is defined as "the capacity or power to do work, such as the capacity to move an object (of a given mass) by the application of force. Energy can exist in a variety of forms, such as electrical, mechanical, chemical, thermal, or nuclear, and can be transformed from one form to another."
2. At the doctors office: When the word energy is used at the doctors office ("how has your energy level been as of late?) the energy being discussed is the "capacity for vigorous activity, available power". The energy for human beings to perform is largely determined by diet, exercise, and amount of sleep. Energy in this context also means the capacity to do work
3.In athletics: Sportscasters with often say things like "you can feel the energy in air" or "look at the energy of the defense." Energy used in this context is defining "the exertion of power".
4. In politics: The phrase "energy policy" comes to mind. Energy policy is a set of rules and regulations used to govern our sources of power. Once again here, energy can be defined as the capacity to do work.
5. In the New age movement: The new age movement is notorious for its overuse of the word energy. The entire movement seems to be based around the idea that there unseen sources of energy all around us. In the new age movement, they of discuss psychic energy, which may or may not exist, this form of energy has been though to be what consciousness is fueled by. Their is always the discussion of bad energy versus good energy in the new age movement as well. Bad energy fuels negative thoughts and emotions, good energy fuels positive thoughts and emotions.
Energy is used in hundreds of other contexts. It seems that the general definition of energy is the force that allows for work to be done. E= MC2 defines energy as mass times the speed of light squared. In the end, it is difficult to really define what exactly energy is.