(feel free to respond to Xan's questions as well, by the way. You seem more in depth than him anyways.)
((CHALLENGE ACCEPTED))
Magic is/would be/was/etc the creation/manipulation/destruction of anything and/or everything via some sort of ritual, which for me right now is a fancy word for 'process'. Most times it takes the form of summoning and manipulating several 'classical' elements, ie fire, water, lightning, etc, but can also take the form of pretty much any supernatural ability: magical shapeshifting where the initiator of the ritual, or 'caster of the spell' transforms into another species very quickly, usually in under half a minute, yet retains their human intelligence despite having no human brain to house it in; various magical curses such as lycanthropy which at predetermined times, ie nights of the full moon, transform the afflicted into an animal ie a wolf or a monstrous form of said animal ie werewolf or something else entirely; summoning rituals/spells/etc that usually call forth residents of another dimension. Most common are demons/angels from the current caster's Heaven or Hell, but various other species are summoned as well, ie fairies, fae, etc, tentacled horrors from the Dungeon Dimensions, or pretty much anything that the caster imagines, as some of these spells summon some magical substance, ie ectoplasm, and shapes it into the shape of the desired creature. I could go on for forever about the various effects that can be seen as magic, but pretty much any effect that is labeled as supernatural is magic. A lot of these magical effects are contained in the form of spells, short rituals and such directed by the craster's willpower, but they can also be in various other forms when 'cast' - words, thoughts, motion, emotional states, pretty much anything really, just depends on the type of magic. There's also magic in the world/that makes up the world that is usually stronger than 'cast' magic. There are additionally various magical creatures like dragons, unicorns, etc. There are also various sentient magical creatures such as vampires, gnomes, etc, but at the higher end of the power spectrum, there are spirits, gods, etc. Depending on the world, it might end up being made out of our type of matter, or magic, or something entirely different. There're also various magical substances like warpstone, lyrium, etc. These are usually made up of solidified magic, usually of another dimension's make. This dimension is usually thr source of magic if that's the case, such as the Fade, the Warp, etc. This dimension may or may not occasionally invade the neighbouring dimension where the players'/readers'/etc perspective comes from. This usually takes the form of demonic incursions, etc. Additionally, with magic comes the concept of souls, an abstract concept, a thing that contains the memories/vital essence/etc of a person and usually goes to another dimension or afterlife after bodily death, or as in the case of Dark Souls, they're more an effect produced by animate creatures that can be consumed when the creature is killed and used to increase one's own soul power, which usually translates into an increase in physical abilities.
There's also more science-themed magic, such as the outright reality warping of the manips in ER, but since it's usually explained through technobabble, it's referred to as 'space magic'. There's also very advanced science in works of fiction that usually is outright magic just disguised as science. This can be seen in Total Annihilation with the nanolathes/etc. These effects aren't more classical magic like fireballs and such, but more esoteric stuff that'd be impossible to do in physics as we understand it.
There's ALSO cases in which magic is a sentient/semisentient force in the universe that it's casters more channel through themselves than actually cast, such as the Force from Star Wars.
To sum up, basically anything that is impossible by our current knowledge of physics could be classified as magic, but it can also be more explicitly labeled as 'magical' forces, such as the aforementioned summoning fore las from nowhere.
Any supernatural effect/entity/etc.