There are two such historical entities:
The first is Carantania (about which I’ve written a lot in my first reply) and its successor Carinthia. Carinthia — more specifically
Gosposvetsko polje / Zollfeld — is commonly perceived as the cradle of Slovenehood and has somewhat of a mythic relevance, the most suitable analogy being with Serbs and (the) Kosovo (field). Minus the chauvinism. :)
The second is Carniola, whose importance grew as the number of Slovene speakers declined in Carinthia. As the only predominantly Slovene crown land within the Habsburg empire — with Ljubljana as its political and cultural centre — it represented the main Slovene administrative division.
Carniolan (kranjski = krainisch) was the synonym for everything Slovene-related (language, people etc.) before that word —
Slovene (slovenski = slowenisch) — itself replaced it. It appeared even outside of Carniola, referring to Slovene things in Styria and Carinthia. This applied as well to how other languages referred to Slovene, for example Russian:
Въ русскомъ языкѣ словенскій до ХІХ вѣка называли краинскимъ/крайнскимъ, виндскимъ, карніольскимъ, карнійскимъ, хорутанскимъ, хорутанско-словенскимъ/хорутано-словенскимъ, крайно-словенскимъ. Съ конца ХІХ вѣка стало преобладать названіе словинскій, затѣмъ вытѣсненною современнымъ словенскій.
From the names listed, one can see that Carantania, Carinthia and Carniola used to be the places that gave relevant descriptive adjectives and proper nouns.
Our flag originates from the national colours (Landesfarben) of the Duchy of Carniola, which, in turn, came from its coat of arms (albeit, there are some disputes regarding the
correct colours and their actual origin). It represented the core of the Slovene identity, many times at the expense of other, non-Carniolan ones.
https://grboslovje.si/shramba20/barve-kranjska.jpg