Argumentum ad populum
Argumentum ad populum (appeal to the people) is a logical fallacy that concludes that a position is true because many or all people believe it. i.e: "If many believe so, it is so." Other names for this are appeal to the masses, appeal to belief, appeal to the majority, appeal to the people, argument by consensus, authority of the many, bandwagon fallacy, and tyranny of the majority.
As an example, homosexuals argue that because same-sex "marriage" is increasingly accepted and indeed has the approval of voters in three states, it must be moral. That argument is fallacious because morality is not determined by a popular vote. As the Catholic Archdiocese of Chicago has noted, if that argument is accepted, "[h]uman dignity and human rights are then reduced to the whims of political majorities."