Appealing to popularity, or the fact that many people do something, as an attempted form of validation.
This is an attempt to prove something by showing how many people agree.
According to a recent Gallup poll, 68 percent of Americans favor teaching creationism in public schools.
Well, that 68% of Americans are ignorant about science, education, and the Constitution.
Ad populum is construed narrowly to designate an appeal to the opinions of people in the immediate vicinity, perhaps in hope of getting others to jump on the bandwagon (which is why this is sometimes referred to as the bandwagon fallacy).
A similar fallacy ad numeram (appeal to numbers) is used to designate appeals based purely on the number of people who hold a particular belief.
Christianity is true. Billions of people can’t be wrong!
Sure they can. At one point in history the entire Middle East and Europe thought the Earth was flat. Whoops!