Argumentum ad populum

Argumentum ad populum (Latin: appeal to the people) is an informal fallacy that argues from the popularity of a position to the truth value of a position. This is widely used in politics: "Surveys show that 90% of voters favor policy X over policy Y, therefore we ought to implement policy X today!"

While the standard argumentum ad populum concludes truth from popularity, the reverse argumentum ad populum has the form "everyone believes X, therefore X is not true". This reverse argument is used quite a bit in marketing by pointing to a popular competitor and saying that their product must be better because it's not popular.