The colophon is a section, usually at the front of a fanzine, containing the editor/publisher's name, publishing house, contact information, issue number, sometimes but not always the date of publication, copyright notice, subscription information and other described ways of getting the publication (see The Usual) and anything else, frequently including whimsy, which the faned may want to put in it.
According to the American Heritage Dictionary, a colophon is an "inscription placed usually at the end of a book, giving facts pertinent to its publication." This is typically such data as the name of the typeface used. As is often the case, fans got it backward — the wrong place, and what most mundane periodicals call a masthead. Faneds use masthead, meanwhile, as entries in Fancyclopedia 1 and Fancyclopedia 2 note, for the large, presentation version of the zine name — what newspapers call a flag.
(There have been many instances where over-enthusiastic neos have neglected to provide either a signed editorial, colophon or a return address, leaving the readers of their fanzines to ponder where subscriptions, contributions and/or letters of comment should be sent, much less who to credit or blame.)