Delusion

A firmly held belief for which there is little or no evidence; or the available evidence contradicts the belief. In order to be classified as a delusion, the belief must not be part of the system of beliefs common to the person's culture.

For example, an American who firmly believes that extraterrestrial beings are visiting earth would not be classified as having a delusion even though there is no good evidence supporting the belief. The belief that alien beings are visiting earth is common in this culture.

On the other hand, an American who believes that extraterrestrial beings live inside of her brain and are causing her to think, feel, and act in particular ways is likely to be classified as having a delusion. This belief is not common in the United States.