What is a rumor? That depends on the context. On Wall Street, a rumor is actionable information of unknown veracity and source. The unknown veracity and source of particular information should, in theory, make that information useless, but on Wall Street, there are rules—laws, in fact—about who gets to tell what to whom and when. Because of these rules, there are some kinds of information that you can only ever get in rumor form.
Some rumors seem to come from out of nowhere, but most arise in response to one of the two great and eternal mysteries of stock trading: Why are shares of XYZ up a noticeable amount today on no news? And… Why are shares of XYZ down a noticeable amount today on no news?