One may find many different definitions of the notion of energy in various sources. Not exactly ”different”, they are all equivalent – it’s the wording that is different. I think that the best textbook definition is a simple one, and I will combine the wording from an Wikipedia article with some extra comments of my own ”additions”:
In physics, energy is the property that must be transferred to an object in order to perform work on or to heat the object, and can be colorlinks, citecolor=black, converted in form, but not created or destroyed.
OK, for me everything is correct in the above definition – with the exception that I would prefer energy be referred to as an ”entity”, rather than as a ”property”. Entity means ”something that exists”, whereas property, according to the dictionary, is ”an attribute, quality, or characteristic of something”. What I don’t like is this ”of something”. The term ”property” in Wikipedia definition implies therefore that energy is always associated with an object – it has to be contained by an object, or transferred from one object to another. In fact, it is ”almost always” the case, but there exist some ”exotic situations” in which it is nor easy to identify ”the object”. For instance, energy – in the form of radiation, such as, e.g., the solar light – is able to travel through empty space. In other words, energy may exist in empty space, or may fill a region of empty space – and is empty space an object? From the viewpoint of advanced modern physics, a firm no! is not a good answer to this question. According to modern theories, vacuum is –let me quote a statement I’ve found in Physics Forum – “very much alive and kicking”. Well, but all this “life” is only a virtual one – we cannot see it, even with the help of sophisticated modern measuring devices. So, if we appeal to our “conventional wisdom”, calling “empty space” an object would be a bit weird. As will also be calling energy flowing through vacuum “a property” of this vacuum. But if instead of “property” we agree to use “entity”, the problem automatically disappears. Energy travels through vacuum, so it does exist in vacuum it travels through – and “entity”, as said above, is “something that exists”. Now everything is OK!
Actually, whether we should say “entity” or “property” is rather a linguistic dilemma, it’s really not so important. One can even think of it as an example of tetraphyloctomy – it’s a Greek term coined by the famous writer Umberto Echo, meaning ”splitting a hair in four”, or paying too much attention in scientific disputes to details that are of little importance, or totally irrelevant. In fact, what is really relevant, is to understand what energy is. Let’s then analyze the Wikipedia definition in closer detail.