: When should we redefine a term that's used in a different way in the book we're writing? Let's say I am writing a fantasy book where magic users doesn't use something called "magic", but use

Obscure Synonyms

Hermetics is a already synonym for alchemy and magic. While it's not an everyday word, anyone looking up "hermetic" in the dictionary will see the alternate definition.

Replace the word with the synonym consistently, and readers will accept it as a worldbuilding substitution that covers the same territory. Don't use the more familiar word ever, or the substitute word suddenly becomes a subset of a broader (unexplained) system.

Hermetic isn't an umbrella term like Magic. As a synonym, it has a narrower definition, in this case a specific culture and time related to Hermes Trismegistus. Most readers, even if they know the word, won't have such a specific definition, but will associate it with medieval Europe and alchemy – your magic system should be congruent with this narrower definition.

A thesaurus offers a list synonyms, each with a different slant.

Euphemisms and compound words

Another way to imply the thing without using the familiar term is to create a euphemism or compound word that suggests the concept through known words: hextongued, wishbound, transmutative science.

This can feel a little heavy-handed since it's a potential pile-up of loaded terms. A skinwalker evokes something more portentous and spiritual than an everyday shapeshifter. A mechanical man evokes a retro-age robot. These things aren't quite the same, and they can seem unnatural in dialog for being too on the nose in their description.

Real language has many words that came into use by combining adjective-noun, but are so familiar that we don't see it. Hedgehogs and a porcupines are both spikey pigs, but a critter named a "spikepig" is too obvious. Firedog is similar, except the dog refers to its shape (low, standing on legs). These words are so familiar we don't see their compound parts, but readers may not be able to unsee that you've stitched descriptive words together.

At the extreme, Star Trek Voyager had a reputation for obfuscating technobabble constructed from long compound words fired rapidly during a crisis. These terms were treated as disposable and were used because they sounded science-y, but had no actual meaning when combined (keyword: babble).

Portmanteau and near-words

Single words that sound close to the actual thing may work so well that people forget they are associated with a specific franchise. Star Trek's phasers are close enough to real world lasers to go unexplained. R2D2 is a droid but not an android (he's not human-shaped). These words are very close to the familiar, and used in such similar context we have no problem grasping their meaning the first time they are used – no explanation necessary.

In fact most fans do not realize the problem "droid" solves by lumping androids like C3PO and all the rollerbots together under 1 umbrella.

Vague (mysterious) words

A vague word can avoid preconceptions, making the thing mysterious and unique to your universe. Where the mundane word is too familiar, a seemingly unrelated word can wipe the slate.

Stephen King's The Shining would not have the same unsettling effect if it was called The E.S.P. or Drinking with Ghosts. These substitutions are too on-the-nose for something that is meant to be unexplainable. The 5yr old protagonist has never met anyone else who can do what he does, and it's disturbing and strange within the story, so when he meets another person who can do it, the older man gives him an explanation intended to put his ability in a happy context:

"You shine on boy, harder than anyone I ever met in my life. […]. You got a knack. […] Me I've always called it shining. That's what my grandmother called it, too. She had it. We used to sit in the kitchen when I was a boy and have long talks without even opening our mouths"

This "exposition" however is not the whole story. This is a description provided by a character, and it doesn't explain it so much as normalize it. He describes it as he understands, but this doesn't explain to the reader how it works. He basically tells the boy "thing exists and special people have it, so you are special." That's actually all the reader needs to understand. King isn't concerned that the reader knows how shining works because the more narrowly it is defined, the less mysterious it becomes.

But "shining" isn't a completely arbitrary word, it hints to the reader an association the characters don't realize yet: the boy is visible and fascinating to malevolent spirits. He "shines" more than anyone else. In this context shining is extremely on-the-nose, but the spirit's motives are so confusing this blatant meaning isn't telegraphed to the reader.

Leaving it unnamed

In the Katherine Bigalow film Near Dark, none of the characters know the word "vampire", even though it follows the usual vampire mythos, strengths and weaknesses. One character asks what they've become, and the answer is "We don't know". By refusing to label it, the story resets the possibilities on the too familiar.

It was an early "sympathetic vampire" story, so rather than carry all the baggage and tropes of all vampire stories ever, it told a smaller story about a few people adapting to strange circumstances and strange bedfellows. It gave an old idea a new slant.

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The problem with your question is that you're asking how to do something that should almost never be done in writing. By establishing this rule in your world "X word has Y meaning," you are locking that understanding in the minds of your readers. Even the best writers struggle to change the understanding their readers have of the meaning of a word or phrase, so instead of fighting an uphill battle, they make distinctions between the terminology. Look at Ancient Zoans, Mythical Zoans, SMILEs, and regular Zoan Devil Fruits in One Piece. Devil Fruit are already distinguished in 3 categories, and Zoan Devil Fruits are broken down into 3 more versions with the artificial Zoans known as "SMILE" as an added "fourth." Instead of trying to change peoples' understanding of how Zoans work, Eichiro Oda just made a distinction in the system.
But let's say you NEED to redefine the term... How might you do that? Look no further than Harry Potter.

"Help will always be given at Hogwarts, Harry, to those who ask for it." I've always prized myself on my ability to turn a phrase. Words are, in my not so humble opinion, our most inexhaustible source of magic. Capable of both inflicting injury, and remedying it. But I would, in this case, amend my original statement to this: "Help would always be given at Hogwarts, to those who deserve it."
~ Albus Dumbledore

Back in the Chamber of Secrets, Dumbledore said the original statement, giving the suggestion that Hogwarts was a safe haven for those who asked for it and that those who are being unfairly persecuted (such as himself at the time) would have the support of Hogwarts, no matter what Death Eaters (or any unjust power) may try to do. In The Deathly Hallows, however, he says this quote where he corrects himself, pointing out that help will come to those who need it, not just those who want it. This gives the implication that no matter how much a villainous person may seek assistance from Hogwarts, they will find themselves with little support. Somebody who deserves help, though, would be able to rally an army to fight alongside them.
So why do I bring this up? Because this is a show of how much weight needs to be put behind a redefinition in order to make it stick. It needs to still be similar in essence, but with a strong enough change to intention that nobody could be confused by what you're now trying to say when using the term/phrase.
For example, Marvel's Thor then Doctor Strange does this well, also.

Your ancestors called it magic, but you call it science. I come from a land where they are one and the same.
~Thor

In this moment, Thor redefines "Magic" within the MCU to mean "Science." A few years later, we get Doctor Strange where we are shown that "Magic" in the MCU still actually can mean Magic, but now we have to fend for ourselves whether it's actually magic or just really advanced science. In fact, they do so in such a way by lampshading the fact that anyone who thinks about this too hard will get confused on how the MCU mechanically functions in regards to "Is it magic or science?"

You think you know how the world works? You think that this material universe is all there is? What is real? What mysteries lie beyond the reach of your senses? At the root of existence, mind and matter meet. Thoughts form reality. This universe is only one of an infinite number. Worlds without end; some benevolent and life-giving, others filled with malice and hunger. Dark places where powers older than time lie, ravenous and waiting. Who are you in this vast multiverse, Mr. Strange?
~The Ancient One

In short, the answer is, if you want to redefine a word, don't. Give the new meaning its own word ("SMILE Fruit") or specification within the old word ("Artificial Zoan", which means the same as "SMILE Fruit"). If you MUST redefine it, do so in a way that is memorable either from a contextual (Harry Potter) position or from a usage (Marvel) position.

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Just redefining a word from everyday usage is rarely good. People will always first associate it with their real-life usage. Maybe switch around a few letters to make it a new "fantasy" word.

About defining the term and the POV: that's really up to you and the POV you are already using.

Some authors prefer to just mention the word often in the beginning when people are talking about it. Like with normal language people will pick it up after a few times and realize that it's supposed to mean. Other authors have some kind of narrator that explains what the "new word" means and what the implications are. That's a pretty easy and straighforward way that is also easy to understand for your readers. You could also have some kind of "teacher explains what it means to his student" moment to have this conversation in-character and only observed by the reader. It really depends on what you are already doing. Whatever you do, try to make it clear that you are currently explaining something to the reader. For example by writing the new word in italics when it first appears.

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