"If there was ever to be a word more puzzling, indefinite, and vague, one might very well declare it nonsense." - Kender ab Annergrave, linguist
Magic, in the simplest terms, is an enigmatic, pervasive force of nature which interacts with and alters the world around it. Simplifying it into a single sentence, however, is a bit of a fool's errand. A person may spend their entire lives studying magic without being able to truly describe it.
Fool I may be, because I will try to do so here.
Physics as we know it is governed by a series of forces like gravity or electromagnetism. These forces interact with each other on a daily basis, and the result is the observable (and inobservable) universe. Think of magic as another one of these forces, acting both as a wave and a particle in the way it interacts with the physical world.
Magic exists everywhere, with different concentrations of particles that exist on different wavelengths. Most "ambient" magic exists on the neutral spectrum and shifts to the negative spectrum as it gains energy and to the positive spectrum as it loses energy.
When high concentrations of ambient magic are subjected to sufficient heat and pressure. they turn into a solid, crystalline form called materia. These materia crystals are natural stores for magical energy, sending it back out when struck like a tuning fork producing a musical tone. Today, materia is used to power magical engines, guns, and other modern devices.
Most organisms possess specialized cells containing mana, a type of organelle used to convert ambient magic into a usable form. After drawing in magical energy, the cells can then re-emit it to produce magical reactions that can do anything from heal wounds to create gouts of flame.
And that is what most people think of when they think of magic- the loud, visible displays of professional magicians that occur every day. But even without recognizing the arcane nature of it, one need only look around at the telegraph wires overhead or the hedges lining the road to realize that magic truly exists everywhere.