Psychosis is usually considered by mainstream psychiatry to be a symptom of severe mental illness. Although it is not exclusively linked to any particular psychological or physical state, it is particularly associated with schizophrenia, bipolar disorder (manic depression) and severe clinical depression.

It is not uncommon in cases of brain injury and may occur after drug use, particularly after drug overdose or chronic use, although certain compounds may be more likely to induce psychosis, and some individuals may show greater sensitivity than others. The direct effects of hallucinogenic drugs are not usually classified as psychosis, as long as they abate when the drug is metabolised from the body.

Chronic psychological stress is also known to cause psychotic states, however the exact mechanism is uncertain. Psychosis triggered by stress in the absence of any other mental illness is known as brief reactive psychosis.

Psychosis is a descriptive term for a complex group of behaviours and experiences and as such is not a medical explanation in itself. Perhaps because of this, it is often confused with syndromes which may seem similar on the surface, or with words which may suggest, or seem to suggest a likeness.

The term psychosis should be distinguished from the concept of insanity, which is a legal term denoting that a person should not be criminally responsible for his actions. Similarly, it should be distinguished from psychopathy, a personality disorder often associated with violence, lack of empathy and socially manipulative behaviour. Despite the fact that both are colloquially abbreviated to 'psycho', psychosis bears little similarity to psychopathy's core features, particularly with regard to violence, which rarely occurs in psychosis, and the distortion of perceived reality, which rarely occurs in psychopathy.