Proof by Assertion
Argument from Repetition · Argumentum ad Nauseam · Argumentum ad Infinitum
A fallacy where a proposition is repeatedly restated regardless of contradiction, and is thought to gain from that repetition.
Origin
The Latin label argumentum ad nauseam — "argument to the point of nausea" — has been used since at least the 16th century; the earliest documented English appearance was in a 1565 translation by John Hall. The fallacy itself was catalogued among classical informal fallacies, and the alternative name "proof by assertion" emerged in modern logic and rhetoric textbooks to describe the same tactic of treating repetition as evidence.
Everyday Use
Ever watched someone keep repeating the same claim in an argument, louder each time, as if saying it again makes it more true? That's proof by assertion. It shows up in political rhetoric, advertising slogans, and family debates — repetition creates a feeling of familiarity that people mistake for evidence.