Circular Reasoning

Circular reasoning is a logical fallacy where instead of the grounds for the conclusion leading to the conclusion the conclusion and the grounds for the conclusion lead to each other.

The circular argument, in which theory and proof support each other (i.e. we repeat ourselves at some point) The regressive argument, in which each proof requires a further proof, ad infinitum (i.e. we just keep giving proofs, presumably forever) The axiomatic argument, which rests on accepted precepts (i.e. we reach some bedrock assumption or certainty) Agrippa's Trilema

Those familiar with some philosophy of mathematics and discussion of circularity in definition will know that not all circles are vicious, so this isn't as hopeless a reply as it might seem. So, you shouldn't assume that to describe reasoning as "circular" is  always  a negative thing (even though it might be in very many instances).

Radical skepticism with regard to the possibility of ultimate philosophical grounding is based on an abstractive fallacy. It is somewhat misleading coherence to present the radical skeptic position in terms of an argument, because in presenting an argument one is usually committed to the truth of its premises and conclusion, whereas radical skeptics would suspend judgment with respect to them. Problems remain regarding the coherence of anyone who accepts the soundness of an argument whose conclusion is that we are not justified in believing anything. The so called Münchhausen trilemma can be overcome by recognizing that some presuppositions are necessary for the very possibility of intersubjectively valid criticism and argumentation. The “principle of fallibilism” which holds that any claim can, in principle, be doubted is only meaningful within an framework where some pragmatic rules and norms are not open to question.

Examples
Darth Dawkins circular reasoning. This Christian apologist is an extremely stubborn, obstinate, and borderline insane presuppositionalist philoso-troll. Like other presuppers, he will shut down any conversation or argument with questions like "how do you know that?" and demands for opponents to justify their ability to reason, but with a level of antisocial animosity at least an order of magnitude higher than your average presupper.

Epistemology Debate About Justified True Belief
 * If someone were to say The Bible is the word of God because the Bible says it's the word of God or if someone were to say Everything is physical so God doesn't exist, God doesn't exist so everything is physical then these would be examples of circular reasoning.