Appeal to belief

Examples:

1. Most people believe in some sort of God, so it must be true.
2. One hundred years ago people didn't believe that smoking was bad for you, so at that time it must have been safe.

Note that under some circumstances, such as social conventions, results of democratic processes etc. this may not be a fallacy. For example:

1. Most people in France believe that it is acceptable for women to go topless at the beach. Therefore it is acceptable in France.
2. Though most people wanted the "small party" to win in election, they did not think it could beat the major parties. This meant that the "small party" did not win.

See also: Communal reinforcement