Well, as a definition of energy I could state something like:
Energy is the ability to do work or to cause change.
Energy is the capacity of a physical system to perform work.
But if you were looking for these you wouldn't be searching here anyway.
definition of energy at Wikipedia, but it tends to be too scientific and involve equations of physics.
I would prefer to use one simple definition of energy, which I was taught at school:
We can only see the results of its application, and that's why you will never find a comprehensive definition of energy to please you.
To sum up, there are different forms of energy:
Those are actually the main forms! I could name some more, but that's not our point right now.
Please have in mind that energy transforms from one form to another and it is never destroyed. This is generally known as the conservation of energy principle.
To Understand that, it is better to use an example again:
Imagine that you go for a ride with your car. You turn it on and it makes noise. After your ride, you park your car at the garage and you notice that engine is hot, and that the fuel is less too.
Here is what happened: The chemical energy (fuel) became kinetic, sound, and thermal energy as well.
Someone could say:
But I do NOT need sound or thermal energy either!
Indeed we loose energy by transforming it in other forms, but this is a necessary evil. What we can do, is energy efficient designs. Those apply not only on machines and vehicles but on buildings as well. We try to make everything use less, and lose less energy.
Well! I was looking something like green energy, or solar power lets say...
I have an answer to that as well. They simply are not forms of energy. These kind of terms refer on the sources where energy is obtained from.
Solar energy for example, is the energy we get from the sun and it is usually either in electric (electromagnetic) or thermal form.
Confused? Lets put things together.
Energy is something we can not see or touch.
We need it to move or shape things.
We get it from different sources.
Use it or store it in different forms.
Haven't told anything about sources of energy yet?
-Conventional Energy Sources
-Alternative Energy Sources
Alternative or renewable energy sources will never be exhausted, whereas conventional sources are fading out day by day.
Need a list? Here it is:
Conventional sources are mostly used at present and actually harm the environment:
Alternative Sources are those which do not harm the environment, and thank God, they will never fade out:
You might say, I DO NOT CARE if it harms the environment, or how i will get it from!
Here is your answer: You may not care but your children do!
Why? because environment is suffering and natural habitat is going to get irreversibly damaged in a few years. We are destroying the earth that our children are going to live on!
So, do you want to do something to change that? Right decision. Think Green on every aspect of your everyday living. Use green - free energy to power up your house and transform it to work in a sustainable way to make our world greener. Everyone can help, and each tiny personal action counts.
P.S. As you can see, there is not such thing as a strict definition of energy. At least not one that everyone understands. The key here is to understand the hole idea and utilise energy as harmlessly as possible.