: Is it a good idea to leave minor world details to the reader's imagination? As a writer, I used to write short stories and poems. As a reader, fantasy is my favorite genre. And I am currently

I think any fantasy story benefits greatly from in-universe terms that have no meaning to a reader. It gives us the very distinct impression we're not in Kansas anymore. But I also think this is a case where showing and telling is appropriate. And you need to be careful, because overuse can be a problem.

Don't just throw out a word. Saying "she dreamt of lillahs" tells me nothing. As you did in your example, adding a normal word or phrase beside it tells me approximately what it is, even if I don't know the details. "She dreamt of lillah birds" tells me what she's dreaming of, while adding that fantasy flair.
Only use such terms where it's useful. Don't just throw them into the text to make things seem alien. In your lillah bird example, the name of the bird is incidental to the fact that your subject is being calmed by listening to them. In the case of the flower pot, it seems more forced. In general, I would only use these terms where it would be important to the character in question.

Bad:

As Adam approached his house, memories of his late wife, Sue, flooded his mind. She had spent so much time caring for the place. Until the cancer set in.

Suddenly, Adam realized he hadn't yet moved from the sidewalk. Pushing the memories away, he hurried up the walkway to the front door. On the way past, he smelled the bettornim flowers that filled the pots along the driveway.… (Here, the flowers are just random details.)

Better:

As Adam approached his house, the scent of the bettornim flowers flooded his mind with memories of his late wife, Sue. She had spent so many hours tending to them in their pots along the driveway, caring for them almost as well as her children. Until the cancer set in.

Suddenly, Adam realized he hadn't yet moved from the sidewalk. Pushing the memories away, he hurried up the walkway to the front door… (Here, the flowers are explicitly acting on Adam, so we feel like they belong to the story.)

If you're going to use the terms more generically, I would tend to use them from a character's perspective, rather than from the narrator's perspective. This makes it feel more like we're hearing these terms because we're in an alien world, and less like the writer is trying to shove fantasy down our throats.

Really Bad:

The man rode up on his unctun with his many dites glittering like thajva gems in the wind. Here, we know the man arrived, and there's something glittering, but we really don't know anything else. Is an unctun a type of animal? Vehicle? Elevator? What about dites, and why do we care how they compare to some gem?

Bad:

The man rode up on his horse, with his many scarves glittering like thajva gems in the wind. Here, we at least know what's going on, and that his scarves are (for some reason we should probably explain better) glittering like something that probably glitters a lot. But the comparison seems a bit forced.

Better:

His purple eyes glittered magnificently in the sunlight -- Tatya likened them to freshly-polished thajva gems. Here, "magnificently" gives the reader Tatya's specific impression, and it is Tatya, not the narrator, likening them to thajva gems.

Selena gushed as she beheld his kingly, purple eyes, "Oh! Your eyes glitter like a thousand thajva gems in the sunlight!" Again, we've added Selena's impression of his eyes rather than simply describe them as "purple", and we're explicitly quoting her in the comparison.

If you're going to use certain terms extensively -- for example, the thajva gem plays a very important part in the local economy -- use a word that's short and easy to pronounce. My examples may not make sense in your native language since you will be used to different sounds than an English-speaker would normally use. I think thajva is a pretty good example itself, though the "jv" letter pairing is a bit weird to an English speaker.

Bad:

The thasagoriaphogleron gems were a staple of the local community, and the group knew they'd need to get their hands on some. "Excuse me, sir," Stephan queried a nearby shopkeeper, "where is the nearest exchange to convert my galactic credits to thasagoriaphogleron gems?" This is just obnoxious, especially if you're going to see the word every other paragraph.

Better:

The thasron gems, claimed by certain uppity scientific circles to be properly referred to as thasagoriaphogleron gems, were a staple of the local community, and the group knew they'd need to get their hands on some. "Excuse me, sir," Stephan queried a nearby shopkeeper, "where is the nearest exchange to convert my galactic credits to thasron gems?" Here, we've put the "proper" term in, given a little more flair to the universe, and made it clear "thasron" isn't the scientific name, but are now free to use a nice word the reader can easily say and remember throughout the rest of the story.

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When in The Lord of the Rings Tolkien writes

‘But it is a heavy burden. So heavy that none could lay it on another. I do not lay it on you. But if you take it freely, I will say that your choice is right; and though all the mighty Elf-friends of old, Hador, and Húrin, and Túrin, and Beren himself were assembled together, your seat should be among them.’ (Elrond addressing Frodo, J.R.R. Tolkien, The Lord of the Rings, book II, chapter 2 - The Council of Elrond)

We have no idea who Hador, Húrin and Túrin are. We only know of Beren as a legendary figure mentioned previously by Aragorn. The others - we can conclude from Elrond's words and from their mention in conjunction with Beren that they were heroes of old. We have no idea what their deeds were.

Tolkien's usage here was neither a mistake nor an accident, but a nod towards the experience of reading an antique text, and experience he was deliberately trying to recreate. Reading an old text, you would sometimes come upon a reference the referent of which has been lost. Other times, the referent is unknown to you yet, and you have to go search and study until you find it. While for the original audience of the piece the referent would have been as well known as The Lord of the Rings is to us today.

One might also experience something similar if one reads a text belonging to a foreign culture, when one isn't very well familiar with the culture in question (yet). Which might be similar to the effect you wish to create with your fantasy/sci-fi story. Or it might not be. Up to you.

Which is to say, using a dead-end reference is a tool you want to use deliberately rather than haphazardly. It creates a particular effect that you might or might not want to use in any given point in your story.

That said, you can vary the measure to which the reference is a dead end. This is discussed well by @ChrisSunami . Consider, we are told at the very least that Hador, Húrin and Túrin are heroes of old. Which is really all we need to know for the passage to work. We get the right image, we don't need the particulars of these men's heroic deeds. This is similar to @Chris 's first two examples.

When reading a real ancient text, dead-end referenced similar to @Chris 's third example also occur, but they offer nothing but frustration. Frustration is not an emotion you'd want to deliberately evoke in your readers. You want to provide some description. The most crucial characteristic for the scene, at the very least.

Something I do not see mentioned here: you want to limit the number of your Thajvas, Gaijamus, Lillahis and whatnot - the more terms your reader has to remember, the less likely they are not to remember them but instead get everything mixed up.

And you don't want to rely on reference notes. They can be a nice supplement for the hardcore fans of your work, but they require extra work from the reader, which means fewer readers would be willing to make the effort.

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My main concern is this part:

I didn't mention these things anywhere else in the novel. Didn't give the reader details about these lilllahi birds, Bettornim flowers, gaijamu flowers, etc.

What I really love about fantasy books, is if I get to remember small recurring elements.

I would not immediatly say what those lilllahi birds look like. But if you mention them five times throughout the book (giving a small detail each time), I might or might not remember that they were mentioned already. However, if I do remember, I will be happy to get a clearer picture, and be proud that I remembered them from earlier.

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Is it a good idea to leave those things to the reader's imagination?

No. This is not good writing, to compare something we know to something we don't know is backwards, at best the reader will mentally reverse it, and try to imagine a flower that looks like a nose, or gems that look like purple eyes.

I wouldn't even count on that, I wouldn't do this AT ALL.

Describe your world in words and images your readers will understand, AND your characters will understand.

If you want to describe something moving fast, but your character inhabits a medieval world, you cannot describe the thing moving as fast as a race car. Not even the narrator can do that.

You must find something that is fast in the medieval world, or you will jerk your reader out of their reverie which is grounded in the medieval world, by reminding them they are reading fiction. What's fast in the medieval world? A race horse, perhaps, a diving hawk, or a runaway wagon flying down a hill.

Don't use made up words for objects (unless you have described them in words the reader will understand), or emotions, or facial expressions or body language.

The same goes for past events: Referencing the horrors of some past war that are never actually described leaves the reader out, even if the other characters seem to understand it. The reader won't adopt their solemnity or shame or whatever they feel, you are telling the equivalent of an inside joke that the reader doesn't get.

The reader needs to identify with the hero (or some character) and they can't do that if the character you intend for them to identify with is reacting to knowledge the reader doesn't have, or to memories or images (like flowers or jewels) that have never been shared with the reader.

Doing what you are doing is keeping the reader at arm's length, not letting them engage with your story, and preventing them from becoming immersed in your story.

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It's okay to have words and alien creatures that your readers don't know about, but if your description cannot teach readers something new or inspire them to picture something, it is empty verbiage and has no purpose. Why should readers care if something looks like a gaijamu flower? Why does the POV care? If nobody cares, there's no point in mentioning it.

If your POV is thinking that something looks like a thajva gem, a visual image will be in his mind, and if we are in his POV we should see it with him. It doesn't have to be more than a mention of what aspects of what he is seeing are like a thajva (brilliant blue, inner sparkling lights, etc)

You might want to ask yourself this: what do I want to accomplish by mentioning this word? If the answer is "nothing" then cut it. It serves no purpose.

It might be that you want to mention the species because you want to show that your world has alien flora and fauna in it. If this is your purpose, it could be accomplished far more effectively by actually having the POV encounter these alien creatures or plants.

If you want to show that the lillahi bird has some sort of special meaning for the character, then let us know what that meaning is and where it came from. Why does it make the character feel calm? Because it is hypnotic and soothing? Or because she used to have them all around her house when she was a child, so hearing the birds takes her back to when she was a child, and makes her feel safe.

If you simply want to describe what the POV is seeing, then describe it. When you are writing about our normal world you can use shortcuts in describing things by comparing them to other things that your readers will be familiar with or have emotional associations with. If you are in a fantasy world, you have to do things the hard way.

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It's perfectly fine to leave details up to the reader's imagination. But those comparisons are neither doing work for you nor for the reader. They have the look and feel of descriptions, but they are empty.

Let's look at some ways of potentially using this technique:

"The trees were full of lillahi birds."

I think this is okay --the reader gets an impression of a tree full of birds, he or she may not to know what those birds look like (although you should know, just in case some detail from that becomes useful to you).

"She reminded me of the lithe, blue lillahi birds that used to flock around my mother's garden.

I think this is also fine. There's enough description and context here that the comparison is actually doing work, but it's subtle and in the background, it doesn't call attention to itself.

"She reminded me of a lillahi bird.

This is where it gets problematic for me. "Lillahi" is a completely empty word for us, it doesn't mean anything. So the reader isn't getting any more information than if you said "she reminded me of a bird." That's just lazy writing. (As John Wu noted in the comments, even this might have a purpose, early in the book, to indicate the exoticism of the narrator, or setting. You don't want to abuse it however.)

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Yes. In fact, it's inevitable that you'll have to leave some details up to your reader's imagination; describing every little detail takes up a lot of space on the page and you can only fit so many pages in a book. Your job is to provide the reader with the most pertinent visuals so they can form the rest of the picture in their mind's eye.

That said, if you're comparing X to Y and I have no idea what Y looks like, I'm going to have a difficult time completing the picture. Consider:

Her beautiful nose looks just like GAIJAMU flowers.

I don't know of any nose-shaped flowers so now I'm imagining a lady with a sunflower for a nose. Unless that's the effect you were going for, you either need to describe what qualities a gaijamu flower and the woman's nose have in common, or find a better description.

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