A self-published magazine or booklet (often by fans) produced by like-minded people, typically containing manga, stories, essays, or fan works.
コミックマーケットで友達の同人誌を何冊か買った。
I bought a few of my friend's dōjinshi at Comiket.
彼は趣味で同人誌を自費出版している。
He self-publishes dōjinshi as a hobby.
その同人誌は人気の二次創作をまとめたもので即完売した。
That dōjinshi, which collected popular fan works, sold out immediately.