The DSM-IV defines a delusion as:

A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everybody else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary. The belief is not one ordinarily accepted by other members of the person’s culture or subculture.

I am inclined to disagree. By that definition, birthers are not delusional, nor are those who believe Obama visited Chicago today to meet with Louis Farrakhan. However, the DSM-IV also goes on to define a delusion as pathological if it is caused as a result of a pathology or a disease process. This then rules out stupidity, misinformation, dogma, or misperceptions.