Boundaries and worlds with

primarily means “boundary” or “a defined area or society”.

Etymology

is a phono-semantic compound built from 田 (field) and 介 as the sound component, with the original sense of boundary lines dividing fields.

Human and Social Worlds

(World)

世界(せかい) primarily has three meanings: the entire globe and all human societies, the world as perceived by humans, and a specific field of expertise or area of activity.

It combines 世 (society) with 界 (world) to show the world of human society.

Manga panel from ARIA The MASTERPIECE showing example of Following after is bad with あかん.
ARIA The MASTERPIECE » Volume 2 » Page 129

Alicia and Akari visit an island that’s modeled after part of Japan back on Earth. There, an elderly local woman tells the two of the fox spirits that visit the human world there. Akari is excited at the prospect of meeting one, but the old woman gives her a warning: on rare occasion, a fox will take a person back home with them.

おばあさん:
(かみ)さまの世界(せかい)人間(にんげん)世界(せかい)(ちが)うやさかい」
“The god world and the human world are different.”
()れて()かれたらあかん」
“Therefore, it is bad to follow along.”
View Full Example

Limits and Boundaries

(Limit)

限界(げんかい) means the absolute boundary, limit, or maximum ceiling of an action, ability, range, or degree, signifying the point where “nothing more is possible” or “there is no going beyond this”.

It combines (limit) with 界 (boundary) to show a boundary line.

Manga panel from あしたは土曜日 showing example of At one's limit with 限界.
あしたは土曜日 » Volume 秋・冬 » Page 16

Mina wants to hurry and eat her lunch, as she’s very hungry. Yukari reminders her they still have one more class first.

ミナ:
「もう限界(げんかい)だよ!こんなんじゃ(つぎ)授業(じゅぎょう)集中(しゅうちゅう)できないよー。」
“I'm already at my limit! As things are, come next class I won't be able to concentrate.”
View Full Example

(Field of view)

視界(しかい) refers to the range that can be seen with the eye, or the clarity of that view.

It combines 視 (look) with 界 (range) to show the range of sight.

Manga panel from ARIA The MASTERPIECE showing example of Your allowance of a rowing technique with 〜させてもらいます.
ARIA The MASTERPIECE » Volume 1 » Page 213

Akatsuki asks Akari for a gondola ride to his destination. Along the way, he asks whether he’ll make his deadline. Akari suggests Alicia should take over, but Akatsuki says he’s Akari’s customer (and paying the cheaper fare). Realizing her responsibility, Akari turns the gondola around so she can use her high-speed backward rowing. Alicia apologizes to Akatsuki for the turnaround.

アリシア:
(いま)からお客様(きゃくさま)視界(しかい)(さえぎ)()(かた)をさせてもらいます」
“From now on, she'll be using a rowing technique that blocks your view.”
(あかつき):
「むぅ?」
“Mm?”
View Full Example

Other Realms

(Passing away)

他界(たかい) is primarily used to politely or indirectly refer to “a person’s death”. It is used to avoid direct expressions in order to ease the shock felt by the bereaved family and to show respect.

It combines 他 (other) with 界 (world) to show the other world.

Manga panel from 名探偵コナン showing example of Photographs of deceased parents with 写真.
名探偵コナン » Volume 26 » Page 139

Kogoro takes a client to the home of someone she’d be communicating with via pagers. They show the household family a music box that was left to the client by her correspondence partner. The music it plays reminds the homeowner of the koto his mother used to play.

小五郎(こごろ):
(かあ)さんってもしかしてあのお写真(しゃしん)の…」
“By 'Mom', could that be the one in that photo…?”
常雄(つねお):
「ええ… (ちち)(よこ)写真(しゃしん)三年(さんねん)(まえ)他界(たかい)した(はは)春菜(はるな)です…」
“Yes... The photo beside my father's is my mother, Haruna, who passed away three years ago...”
View Full Example

(Other world)

異世界(いせかい) refers to a different space or dimension distinct from the real world in which we live. In modern entertainment, it often refers to fictional fantasy worlds where magic, swords, and monsters exist.

It combines (other) with 世界 (world) to show another world separate from ordinary reality.

Manga panel from キラキラとギラギラ showing example of In another world with 異世界.
キラキラとギラギラ » Volume 1 » Page 173

Zen lived in a glittering, idyllic town until his family moved. During his first day of school, he found everyone to be glaring and rugged. Rather than being gentle and playful, his classmates sought a brawl and chased after him.

(ぜん):
「ここは異世界(いせかい)だ」
“This is another world.”
異世界(いせかい)(まよ)()んだに(ちが)いない」
“I must have wandered into another world.”
View Full Example