A delusion is a false fixed belief that is not amenable to change in light of conflicting evidence. As a pathology, it is distinct from a belief based on false or incomplete information, confabulation, dogma, illusion, hallucination, or some other misleading effects of perception, as individuals with those beliefs are able to change or readjust their beliefs upon reviewing the evidence. However:
"The distinction between a delusion and a strongly held idea is sometimes difficult to make and depends in part on the degree of conviction with which the belief is held despite clear or reasonable contradictory evidence regarding its veracity."
Delusions have been found to occur in the context of many pathological states (both general physical and mental) and are of particular diagnostic importance in psychotic disorders including schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.
Delusions are categorized into four different groups:
Bizarre delusion: Delusions are deemed bizarre if they are clearly implausible and not understandable to same-culture peers and do not derive from ordinary life experiences. An example named by the DSM-5 is a belief that someone replaced all of one's internal organs with someone else's without leaving a scar, depending on the organ in question.
Non-bizarre delusion: A delusion that, though false, is at least technically possible, e.g., the affected person mistakenly believes that they are under constant police surveillance.
Mood-congruent delusion: Any delusion with content consistent with either a depressive or manic state, e.g., a depressed person believes that news anchors on television highly disapprove of them, or a person in a manic state might believe they are a powerful deity.
Mood-neutral delusion: A delusion that does not relate to the patient's emotional state; for example, a belief that an extra limb is growing out of the back of one's head is neutral to either depression or mania.
In addition to these categories, delusions often manifest according to a consistent theme.