Circular Reasoning
Circular reasoning happens when an argument assumes the truth of the conclusion it is supposed to establish. Instead of moving from evidence to claim, it loops back and quietly uses the claim as one of its own supports.
The obvious form sounds silly. The dangerous form sounds respectable. It often appears inside tradition, institutional design, legal categories, and repeated assumptions that no longer feel like assumptions. "This must remain the way it is because this is what it is" is the underlying structure, even when the wording is more polished.
The practical test is simple: if you remove the conclusion from the premises, is there still an independent reason left? If not, the argument has only restated itself. Circular reasoning is repetition dressed as justification.
Sources
See also: critical-thinking-and-logical-fallacies for how this fallacy fits alongside the full pattern of argument failures.