Circular reasoning
Etymology
The Latin name "circulus in probando" means "a circle in proving," also called a "circulus vitiosus," a vicious circle. The picture is of a chain of reasons that curves back to where it began. However many steps it takes, no independent ground is ever touched.
noun phrase · /SUR-kyuh-ler REE-zuh-ning/
Supporting a claim with reasons that ultimately depend on the claim itself.
What it names
Catching it exposes arguments that travel in a loop and never actually reach outside themselves for support.
Example
"He’s trustworthy because he says so, and he wouldn’t lie because he’s trustworthy" is circular reasoning.
Related idea
Begging the question is the same flaw compressed into one step, assuming the conclusion directly in a premise.