Azuma

...Furthermore, in the late Jomon period, the Yayoi culture, which began with the spread of rice cultivation techniques to northwestern Kyushu, spread rapidly throughout western Japan, while eastern Japan remained under the Jomon culture for some time, making this difference even more striking.
[The area of ​​the Eastern Provinces]
In Western Japan, a state based on rice paddies was formed before in Eastern Japan, and eventually a government based in Kinai expanded its influence over the main part of the Japanese archipelago, and Eastern Japan, except for northern Tohoku, came under its influence. The people who made up this government called Eastern Japan "Azuma" or "Eastern Japan," which primarily meant the Kanto and Tohoku regions. However, in terms of the area covered by "Azumauta" (Eastern Song) and the area where Sakimori (Border Guards) were mobilized, Eastern Japan is even broader, and often refers to the area east of Echigo, Shinano, and Mikawa, which roughly corresponds to Eastern Japan in the Jomon and Yayoi periods, and in fact this region was different from Western Japan in terms of language and folklore.

*Some of the terminology explanations that mention "Azuma" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…そして縄文晩期，西北九州に稲作の技術が伝播したのを契機に始まる弥生文化が西日本に急速に広がったのに対し，東日本がしばらく縄文文化にとどまったことは，この差異をさらに著しいものにした。
［〈東国〉の範囲］
　西日本には水田を基礎とする国家が東日本に先んじて形成され，やがて畿内を基盤とする政権が日本列島の主要部に影響を拡大し，東北北部を除く東日本もその影響下に入るが，この政権を構成する人々は，東日本を〈あづま〉〈東国〉といっており，それはまず関東･東北地方を意味していた。しかし〈東歌（あずまうた）〉のとられた範囲，防人（さきもり）の動員された地域からみると東国はさらに広く，縄文･弥生時代の東日本とほぼ一致する越後･信濃･三河以東の地域を指すことが多く，事実この地域は言語，民俗においても西日本と異質なものをもっていた。…

※「あづま」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報