What does 同人誌 Doujinshi mean?

Doujinshi is self-published work. The term specifies how something reached its readers, and says nothing whatsoever about its subject, its quality, or whether it borrows characters from anyone.

The three characters

同 (dō) is same. 人 (jin) is person. Together 同人 means people of the same interest, a circle formed around a shared enthusiasm, and the term is older than manga: literary coteries publishing their own journals used it well before comics existed. 誌 (shi) is a periodical.

So the literal sense is a magazine put out by a group who share an interest, which is a description of a publishing arrangement and not of a genre.

What the category actually contains

Original comics and fiction by people without a commercial contract. Criticism and essays. Illustration collections. Research and non-fiction on subjects too narrow for a publisher, including a great deal of serious work on trains, history and other specialist enthusiasms. And yes, a large volume of work using other people's characters, some of it explicit.

The last of those is real and is not the definition. It is a subset that happens to be the subset that travelled.

How English narrowed the word

By sample, in the same way seinen came to mean grim. The material that reached English-speaking readers early arrived through informal channels that were not selecting for representativeness, and the explicit parody subset was heavily over-represented in what got scanned, translated and circulated.

English then took the word from that sample. The result is a term that means self-published in Japanese and means something considerably more specific in English, with the speakers of each convinced they are using the same word.

Why it matters more than a translation quibble

The doujin scene is not a fringe of Japanese comics; it functions as a training ground and a talent pipeline, and a number of professional careers began in circles selling photocopied books at conventions. Publishers largely tolerate derivative work rather than pursuing it, an arrangement with no exact equivalent elsewhere, and one that only makes sense if the scene is understood as an amateur publishing culture rather than as an infringement problem.

Reading the word as fan pornography makes that arrangement incomprehensible. Reading it as self-publishing makes it obvious.

The nearest English word, and why it faded

Fanzine is the closest, and it has aged into obsolescence along with the mimeograph. English publishing has since absorbed self-published as a neutral description, but that phrase carries no sense of a circle, no sense of a convention where the work is sold hand to hand, and no sense of a scene with its own economy. Those are precisely the parts the Japanese word keeps.