>>261537Doujin means it's self-published, i.e. at cons or other events. Can be doujin music, doujin games, or doujinshi (comics).
A commercial manga sold in stores can't be called a doujinshi unless you routinely call e.g. the Batman comic a fanzine. Or Lord of the Rings a homemovie.
tl;dr every doujinshi is a manga, but not every manga is a doujinshi