Arguing that something is true, correct, or desirable primarily because many people believe or do it. Popularity is substituted for evidence.
"Seven out of ten Americans support this policy — isn't it time you did too?" Popular support is real data about public opinion, but it's not evidence about whether a policy works. Majorities have supported things that were scientifically, morally, or practically wrong.
Humans are social animals. Disagreeing with an apparent majority triggers mild social anxiety — the sense of being out of step, isolated, or wrong. The bandwagon appeal exploits this to make agreement feel like the path of least cognitive resistance. The argument structure is: "Lots of people believe X, therefore X." This is invalid as logic but deeply effective as persuasion.
Arguments that lead with polls, consensus figures, or phrases like "most people agree" or "everyone knows" without any substantive evidence following.