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I have read The Light Fantastic By Terry Pratchett in English and always interpreted title as "The Fantastic(adjective) light(noun)". Mostly because I do not see "Fantastic" as noun. To my surprise the localized (Slovak) version has title "Ľahká fantastika" translatable as "The Light(adjective) Fantastic(noun)", "Easy fiction" or "The Lightweight Fiction".

Is "The Light(adjective) Fantastic(noun)" correct meaning/translation of title? If yes, does "Fantastic(noun)" here mean fiction?

Investigating this only made me more confused:

Reading the wikipedia page, title of the book is quote from "L'Allegro" by John Milton, but there it is used like this:

Come, and trip it, as you go,
On the light fantastic toe; And in thy
right hand lead with thee

So there it is used as "The Light(adjective) Fantastic(adjective)"?.

Definition for "Fantastic(noun)" yields "A fanciful or whimsical person." so this could refer to one of MCs of the book (Rincewind?).
I hope this fits here better than English SE.

In-universe the "light fantastic" is an actual, factual thing.

There was no real need for the torches. The Octavo filled the room with a dull, sullen light, which wasn’t strictly light at all but the opposite of light; darkness isn’t the opposite of light, it is simply its absence, and what was radiating from the book was the light that lies on the far side of darkness, the light fantastic.

It was a rather disappointing purple color.

_{^{The Light Fantastic}}

Pratchett, in the Discworld Companion notes that this is not to be confused with Octarine (which is the "Colour of Magic") and splits from ordinary light.

Light, nature of.

Meta-light is almost an idea rather than a phenomenon. It is the light by which darkness can be seen, and therefore is always available, everywhere. If it didn’t exist, darkness could not be visible. It is widely used in the film industry for shots in caves and mines.

...

The light fantastic is perhaps best evidenced by the dull, sullen light which fills the room where the OCTAVO is kept. Not strictly light at all but the opposite of light. Darkness is not the opposite of light, it is simply its absence. The light fantastic is the light that lies on the far side of darkness.

Ordinary light passing through a strong magical field is split into not seven but eight colours, and the eighth – OCTARINE – is generally associated with things magical. It can be described in terms of other colours about as readily as red can be described in terms of green, yellow and blue, but if some description is really insisted on then octarine is a rather disappointing greeny-purple-yellow colour.

As far as can be determined, there are now four distinct types of
light on the Disc. For the sake of discussion they could be called
common light, meta-light, dark light and ‘the light fantastic’.

It is a punning reference to the phrase ‘trip the light fantastic’, which means (per The Phrase Finder)

To dance, especially in an imaginative or 'fantastic' manner.

The phrase seems to arise from the works of Milton, in Comus he wrote, as you have already seen,

Come, knit hands, and beat the ground,
In a light fantastic round.

And in L’Allegro

Come, and trip it as you go
On the light fantastic toe.

Later, in 1894, as cited in Wikipedia, Clarke’s B Lawlor and James W Blake included the lines

Boys and girls together, me and Mamie O'Rourke

Tripped the light fantastic

On the sidewalks of New York.

So, as summarised by WordOrigins.com

It is not light that is fantastic, but rather the toe or dance step. Both trip and light refer to the movement of the feet.

And this is where the pun comes in, Pratchett uses the phrase to refer to actual ‘light’ that is ‘fantastic’. In this sense it is both the light cast by the red star and the star itself which can be referred to as the ‘fantastic light.’

The translated title's doubled meaning is therefore a fortuitous co-incidence which preserves the spirit of the original punning reference and is an example of how good translation goes beyond the mere literal substitution of words.

The light that Pratchett refers to is Octarine.

This is defined in the Discworld books as the eighth colour of the spectrum and the colour of magic. "The Colour Of Magic" itself being a title of another book in the series.

This is fantastic because its existence is part of the Discworld fantasy universe. Pratchett is very fond of such puns and allusions in his work.

So the Light Fantastic is indeed noun - adjective, a poetic syntax.

And to add yet another facet: "light" as a distinctive qualifier is used in military, such as in the famous "The Charge of the Light Brigade" poem by Alfred, Lord Tennyson or "Leichte Kavallerie" (light cavalry), an obscure operette by Franz von Suppé about a dance troup that is these days only known because of its world famous ouverture. So "The Light Fantastic" can also allude to an agile form of fantasy.