Presume Facts Not Yet Proven

In short, any time an argument is based on facts or evidence that have yet to be demonstrated or proven.

For example, let's say that Kathy has a book that she says proves the existence of Valerie, Queen of the Rocky Mountain Fairies. Jennifer says she doubts the book's contents are true, and says it's probably either a hoax or a work of fiction. Kathy turns to page 14 and quotes part of the book: "...But I, Herbert Robertson, hereby do declare that everything in this book is true!" She therefore concludes that the book must be true. But to use this part of the book as evidence that the rest of the contents are true presumes that Herbert Robertson actually existed and was not a creation of whoever wrote the book, and that if he did exist, he was being honest - neither of which have been demonstrated or proven yet. Herbert Robertson could be a fictional character, or just a liar - in which case, his statement that the contents of the book are true don't prove anything.

Another example of this fallacy would be "It's wrong because it's illegal. If it weren't wrong, it wouldn't be illegal." This presumes that things that aren't necessarily wrong have never been illegalized.

Now, one thing that is not this fallacy is a circular definition, EG, "it's a circle because it's perfectly round; it's perfectly round because it's a circle" or, "it's broccoli and cheese; it's made of cheese and broccoli." Circular definitions can be very unhelpful - as they are built on terms or definitions that rely on each other to explain, they explain nothing to anyone who is unfamiliar with neither term or definition. So, they aren't necessarily fallacious so much as obnoxious.