What happens if you start writing a book, or perhaps a small series of books, and it blossoms into a monstrosity beyond your control, getting lost in irrelevant subplots, minor characters, and the enjoyment of making money listening to yourself talk.
The name comes from Robert Jordan, of The Wheel of Time fame. It was meant to be but a few books long, and the first several were good (I started to get incredibly tired of the series starting in A Crown of Swords, but A Path of Daggers really lost me). The first several books each took place over a year or so of time. But as the books grew in number, the books took place over smaller amounts of time, as RJ started to pay too much attention to unimportant storylines and hundreds of irrelevant minor characters. Over half of Book 10 (Crossroads of Twilight) took place over about a 12 hour period (through many points of view), which coincided with Book 9's timeline! Even his most rabid fans are sick and tired of his writing, and almost all have abandoned the series for greener pastures (see A Song of Ice and Fire).
Although this terrible affliction is named after Robert Jordan, in no way is he the first sufferer. The most notable one might be J. R. R. Tolkien, although it is difficult to compare the two. JRRT was obsessed with his own worlds, and was so dedicated to them that he barely got around to publishing much of them. Even The Lords of the Rings is a bit dry and long-winded, but The Silmarllion is a great example of creeping Jordanitis.
One thing that many are fearing is that George R. R. Martin will come down with this terrible disease, with his series, A Song of Ice and Fire. One of the primary symptoms, that of a short series blossoming into a rather long series, has already occurred (the original size has doubled, from 3 to 6). However, so far, his books have done nothing but improve in quality, with intricate plots and detailed characters, and I, for one, would not mind seeing many more of them.