What is the appeal to popularity / bandwagon fallacy (ad populum)?

Concluding a claim is true because most people believe it or approve of it — popularity is not proof.

The pattern: most people approve of X; therefore X is true. It fails because a majority can be wrong — at various times most people held that the Earth was flat or the centre of the universe. Number of believers is not evidence about reality.

"Everyone's switching to this diet, so it must really work."

Lots of adopters tells you it's fashionable, not that it's effective. Closely related cousins: appeal to common practice ("everyone does it, so it's okay"), peer pressure (conform or be rejected), and appeal to belief (most people believe X). All substitute headcount for evidence.

Tip: Distinguish from cases where consensus is meaningful — e.g. a strong scientific consensus reflects converging evidence, not mere popularity. The fallacy is treating bare numbers, with no evidence behind them, as the proof.