Paranoia is a disturbed thought process characterized by excessive anxiety or fear, often to the point of irrationality and delusion. Paranoid thinking typically includes persecutory beliefs concerning a perceived threat towards yourself. In the original Greek, παράνοια (paranoia) simply means madness (para = outside; nous = mind). Historically, this characterization was used to describe any delusional state.
Sometimes in common usage, the term paranoia is misused to describe a phobia. For example, a person may not want to fly out of fear the plane may crash. This does not in itself indicate paranoia, but rather a phobia. The lack of blame in this case usually points to the latter. An example of paranoia, however, would be fear that the pilot is an alcoholic with no evidence to suggest such, and would crash the plane as a result of this.
Paranoia is often associated with psychotic illnesses, sometimes schizophrenia, although attenuated features may be present in other primarily non-psychotic diagnoses, such as paranoid personality disorder and obsessive compulsive disorder. Paranoia can also be a side effect of medication or recreational drugs such as marijuana and particularly stimulants such as methamphetamine and crack cocaine. In the unrestricted use of the term, common paranoid delusions can include the belief that the person is being followed, poisoned or loved at a distance (often by a media figure or important person, a delusion known as erotomania or de Clerambault syndrome). Other common paranoid delusions include the belief that the person has an imaginary disease or parasitic infection (delusional parasitosis); that the person is on a special quest or has been chosen by God; that the person has had thoughts inserted or removed from conscious thought; or that the person's actions are being controlled by an external force. Therefore, in common usage, the term paranoid addresses a range of mental conditions, assumed by the use of the term to be of psychiatric origin, in which the subject is seen to generalise or project fears and anxieties onto the external world, particularly in the form of organised behaviour focused on them. The syndrome is applied equally to powerful people like executives obsessed with takeover bids or political leaders convinced of plots against them, and to common people who believe for instance that shadowy agencies are operating against them.
The term paranoia was used to describe a mental illness in which a delusional belief is the sole or most prominent feature. In his original attempt at classifying different forms of mental illness, Kraepelin used the term pure paranoia to describe a condition where a delusion was present, but without any apparent deterioration in intellectual abilities and without any of the other features of dementia praecox, the condition later renamed schizophrenia. Notably, in his definition, the belief does not have to be persecutory to be classified as paranoid, so any number of delusional beliefs can be classified as paranoia. For example, a person who has the sole delusional belief that he is an important religious figure would be classified by Kraepelin as having 'pure paranoia'.