What does 異世界 Isekai mean?

Isekai means another world, and it names stories in which someone from this one ends up there. The protagonist is typically summoned, reincarnated, or killed in traffic, and wakes somewhere governed by unfamiliar rules.

The characters

異 (i) means different, other, foreign; it appears in words like 異文化 (ibunka, a different culture) and 異常 (ijou, abnormal). 世界 (sekai) is world. The compound is exactly as plain as it looks.

This one really is a genre

Worth stating because it contrasts with shonen, which English fandom treats as a genre and which is not one. Isekai is different: Japanese publishing, bookshops and readers use it as a content category, describing what happens in the story rather than who is expected to buy it. When a Japanese reader calls something isekai they mean the same thing an English reader does.

Where the boom came from

Not from anime studios. It came from Shosetsuka ni Naro, a free web-novel platform where anyone can serialise, and where reader rankings are visible and immediate. Isekai premises did well there for structural reasons: a protagonist who arrives knowing nothing lets the author explain the world naturally, and a familiar setup lets a new writer skip straight to the part readers came for.

Successful web novels were picked up as light novels, then as manga, then as anime. That pipeline is why so many series share not just a premise but a texture, including status screens, levels and skill lists. Those are not references to games in a loose sense. They are the interface conventions of a platform where the audience were themselves gamers, hardened into genre furniture.

The criticism, and the fair answer to it

The standard complaint is that isekai is wish fulfilment: an ordinary person is transported somewhere their ordinariness becomes exceptional, usually with cheat-level powers supplied on arrival. That is accurate about a large part of the output.

The fair answer is that the premise is old and the genre is not defined by its laziest members. A Connecticut Yankee in King Arthur's Court is isekai. So is The Wizard of Oz, and Narnia, and Spirited Away, which is a portal fantasy in which the protagonist is handed no power at all and has to work in a bathhouse. The structure supports more than the boom typically asks of it.

Why English borrowed a word it already had

English has portal fantasy, and it is an accurate description. It is also an academic-sounding term almost nobody says aloud, coined for criticism rather than for conversation. Isekai arrived attached to a specific, recognisable, contemporary body of work, and it is three syllables. Convenience decides these things more often than necessity does.