What is begging the question (circular reasoning / petitio principii)?

Assuming the very thing you're trying to prove — the conclusion is smuggled into the premises, so the argument proves nothing.

The pattern: the premises already include or presuppose the conclusion. It fails because assuming a claim is true is not evidence that it's true — the argument just travels in a circle and never connects to anything outside itself.

A: "How do we know the book is reliable?" B: "Because it says right here that everything in it is true."

The book's reliability is being supported by an assertion that only counts if the book is already reliable. Blatant version: "X is true; the evidence is that X is true." Subtle versions hide the loop behind rephrasing.

Tip: Note the modern colloquial use of "begs the question" to mean "raises the question" is different from this logical sense. The fallacy is specifically circular support.