Is isekai the same as transmigration?

Not exactly. Isekai means being transported into a different world, often as a new or altered self. Transmigration means taking over an existing character's established identity, usually within a story the protagonist already knows.

What is transmigration in web novels?

It's when a protagonist's consciousness ends up in the body of an existing character from a book, show, or game — living out that character's life with outside knowledge of what happens next.

Which is more popular, isekai or transmigration?

Both are extremely popular in web fiction, though isekai has broader mainstream anime recognition while transmigration is especially dominant in fanfiction, since it works naturally with existing source material.

Isekai vs Transmigration: What's the Difference?