A publisher or printer (or binder) 'imprint' containing details on the publication of a work. Usually this will mention the date of publication and the name of entity doing the publishing (circle name and printing company name, for most doujin work). A colophon is often found near the end of a published book or doujin, at the bottom of a page.
For doujinshi 'fanzines', this page is often a dedicated credits page. Occasionally the colophon may instead be found in the afterword page, the (back) cover, or included at the bottom of a comic page.