WHAT IS PARANOIA?

Paranoia is an excessive anxiety or fear concerning one's own well-being which is considered irrational and excessive, perhaps to the point of being a psychosis. This typically includes persecutory beliefs concerning a likely threat, or a belief in a conspiracy theory. In the original Greek, παράνοια (paranoia) means simply madness (para = outside; nous = mind) and it is this use which was traditionally used in psychiatry to describe any delusional state. However, the exact use of the term has changed over time in medicine, and because of this, modern psychiatric usage may vary. Paranoia is distinct from phobias where there is an irrational and persistent fear of certain situations, objects, animals, activities, or social situations. By contrast, the paranoid person blames and/or fears intelligent beings for their supposedly intentional actions.

Use in psychiatry
In psychiatry, the term paranoia was used by Emil Kraepelin 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, Emil 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'.
Although the diagnosis of pure paranoia is no longer used (having been superseded by the diagnosis of delusional disorder) the use of the term to signify the presence of delusions in general, rather than persecutory delusions specifically, lives on in the classification of paranoid schizophrenia, which denotes a form of schizophrenia where delusions are prominent.
More recently, the clinical use of the term has been used to describe delusions where the affected person believes they are being persecuted. Specifically, they have been defined as containing two central elements:

1-The individual thinks that harm is occurring, or is going to occur, to him or her.
2-The individual thinks that the persecutor has the intention to cause harm.

Paranoia is often associated with psychotic illnesses, particularly schizophrenia, although attenuated features may be present in other primarily non-psychotic diagnoses, such as paranoid personality disorder. Paranoia can also be a side effect of medication or recreational drugs.

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 projects 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 insignificant people who believe for instance that shadowy agencies are operating against them.

Paranoia depicted in popular culture
In popular culture paranoia is often represented as including:
Belief in having special powers or being on a special mission (a "delusion of grandeur")
Conspiracy theories, such as seeing seemingly unrelated news events as parts of a larger, typically conspiratorial plan
Exaggerated fear of terrorists, criminals or bandits
Black helicopters and other mass surveillance
Persecution from powerful adversaries such as UFOs, terrorists, the Men in Black, secret societies or demons
Paranoia or belief in conspiracy theories involving perceived political or social enemies (as in The Paranoid Style in American Politics by Richard Hofstadter)
Mind control through invisible rays, and tinfoil hats to combat them
Fear of poisoning, adulterated food (e.g., aspartame) or water (e.g., fluoridation) as part of a secret plot. In the film, Dr. Strangelove, the character Brig. General Jack D. Ripper suffers from paranoid delusions regarding fluoridation.

Reading a story, watching a movie, or listening to a song and feeling that one's life is exactly like that of the subject of said story, movie, or song. The movie The Truman Show, which depicted a man who discovers his entire life has been filmed as a TV show, is one of the more commonly referenced films.

In the Tremors series of films, the character Burt Gummer (Michael Gross), is remarked by the other characters as paranoid, owing to his extreme distrust of the government, fear of eminent domain, general suspiciousness of his fellow human beings.
The maxim: Just because you're not paranoid doesn't mean that they aren't really out to get you. Along the same vein: Even paranoids have enemies, Aand "No matter how paranoid you are, the government is involved in conspiracies even worse than you think."
The Douglas Adams Infocom game Bureaucracy contains the "Popular Paranoia" newspaper that attracts potential readers with the line "Isn't it time you gave yourself eyes in the back of your head?"