Delusion (de-lu-sion )

An abnormal state in which a person has a persistent, false belief that is maintained despite indisputable evidence to the contrary. This belief may be with regards to his own self, other people or any object.

An inability to distinguish between what is real and what only seems to be real.

E.g. A person may falsely believe that a rope lying in front of him is a snake despite everybody trying to convince him that it is only a rope.