topic driftn. Term used on GEnie, Usenet and other electronic fora to describe the tendency of a thread to drift away from the original subject of discussion (and thus, from the Subject header of the originating message), or the results of that tendency. The header in each post can be changed to keep current with the posts, but usually isn't due to forgetfulness or laziness. A single post may often result in several posts each responding to a different point in the original. Some subthreads will actually be in response to some off-the-cuff side comment, possibly degenerating into a flame war, or just as often evolving into a separate discussion. Hence, discussions aren't really so much threads as they are trees. Except that they don't really have leaves, or multiple branching roots; usually some lines of discussion will just sort of die off after everyone gets tired of them. This could take anywhere from hours to weeks, or even longer.