How to Say "Domain Expansion" in Japanese

If you mean the Jujutsu Kaisen battle technique, it is 領域展開 (ryōiki tenkai) — a fixed four-character term used as a shout or as a noun. For a non-anime sense like expanding a business or technical domain, Japanese uses different wording such as 領域の拡大.

The most common way

ryōiki tenkai

The name of the ultimate technique in Jujutsu Kaisen: a sorcerer manifests their innate cursed technique as a closed barrier space. Used as a shouted declaration or as a plain noun.

How to say it: Say 領域 as four clear beats, ryo-o-i-ki — do not squeeze the い out and turn it into "ryoki". 展開 (ten-kai) is flat, with no strong stress on either half.

Other ways to say it

manga / emphatic
!
ryōiki tenkai!
The same phrase with a small ッ, the manga convention for a clipped, explosive shout — how a character actually yells it mid-fight.
When to use it: When you are imitating or quoting the anime dramatically, or joking around with friends who know the series.
casual fan shorthand
ryōiki
Just "domain" — fans drop 展開 in fast conversation and it still reads as the technique, roughly like English fans saying "he popped his domain".
When to use it: Among friends who already know the topic, in a quick comment like "あのキャラ、領域持ってる?".
explanatory / sentence use
ryōiki o tenkai suru
To expand one's domain — the verb form, used when explaining or describing the action rather than shouting it.
When to use it: When telling someone who has not seen the show what a character does, or writing about the series.
business / literal
ryōiki no kakudai
The expansion of a field or scope — the ordinary, non-anime way to talk about widening a business, research or territorial domain.
When to use it: In a work or academic context when you literally mean broadening a field of activity; 領域展開 would be misunderstood there.

In a conversation

A
!
Ryōiki tenkai!
Domain Expansion!
B
、。?
Ō, kakkoii. Nan no mane?
Whoa, cool. What are you imitating?
A
。
Jujutsu kaisen no hissatsu waza da yo.
It's the finishing move from Jujutsu Kaisen.
B
、。
Naruhodo, anime no yatsu ne.
Ah, got it — the anime thing.

Common mistakes

~~ドメイン展開~~ → 領域展開
Japanese does not swap the English word "domain" into this term; the set phrase uses the Sino-Japanese word 領域.
~~領域を展開!~~ → 領域展開!
As a battle cry it is a fixed compound noun shouted bare — adding を turns it into a sentence fragment, which loses the punch.