Delusion: Belief in a falsehood.
There are more comprehensive definitions in psychiatric handbooks, that may refer to the difficulty of removing a delusion, but the core of it all is as defined: One believes what is not so, while not knowing and not believing one does, and the more fanatical or prejudiced one is, the more deluded one is. (See illusion.)
Note that there are several possible reasons for delusions, such as:
Sometimes several of these operate at the same time.
It is the fate of human beings to be deluded about many things, since one is born with very little knowledge, and there is very much to learn and to know, nearly all of which cannot be crammed in one human life. Besides, many delusions of very intelligent men and women were and are due to simply lacking the knowledge to arrive at true or adequate explanations.