Psychosis is a general medical term for a change in how a person experiences reality, and understanding it clearly is more useful than fearing the word.
What the word covers
"Psychosis" is not a diagnosis in itself. It is a general term clinicians use to describe a set of experiences in which a person's perception of reality shifts, often without them realising it has happened. These experiences can include hearing, seeing or otherwise sensing things that other people do not (hallucinations), or holding beliefs that feel completely certain but are not shared by anyone around the person and are not open to ordinary persuasion (delusions). Thinking can also become harder to follow, for the person and for others listening. The nature and content of these experiences vary widely from person to person, and can also shift over the course of a single episode, which is why psychosis is better understood as a spectrum of experience rather than a single fixed state.
Psychosis can appear as part of several different mental health conditions, including mood disorders, and it can also occur on its own, briefly, in response to extreme stress, sleep loss or substance use. It is not the same thing as having a "split personality", a common misunderstanding that confuses psychosis with an entirely different and much rarer condition. Nor does it mean a person is dangerous; most people who experience psychosis are far more likely to be frightened or withdrawn than to harm anyone else.
Why it happens
There is no single cause. Vulnerability to psychosis appears to arise from a mix of genetic inheritance, early development, and the brain's stress-response systems, interacting with life events and circumstances such as isolation, trauma or substance use. This is often described as a stress-vulnerability model: some people carry a higher underlying vulnerability, and psychosis becomes more likely when that vulnerability meets enough pressure. It is a useful reminder that psychosis is not a matter of weakness or character, and that the same person can move in and out of an episode as circumstances change. Researchers are still working out exactly how these factors combine, and no single test currently identifies who will and will not experience psychosis in advance; vulnerability is a matter of probability, not certainty, for any individual person.
What tends to help
Because psychosis has many possible contributors, care usually draws on more than a single approach at once. Antipsychotic medication can reduce the intensity of hallucinations and delusions for many people, while talking therapies help someone make sense of what has happened and rebuild confidence in their own judgement. Practical support, such as stable housing, a steady daily routine and contact with people who are patient rather than alarmed, tends to matter just as much as any single treatment. Family members and friends often benefit from information and support of their own, since living alongside someone in an episode of psychosis can be frightening and disorienting. Ongoing monitoring and a trusted relationship with a care team also matter, since psychosis can change in intensity over time, and early adjustments to a care plan tend to work better than waiting for a crisis to force the issue.
Recovery is common, and for many people psychosis is a period rather than a permanent state. Anyone worried about themselves or someone close to them should talk to a clinician rather than try to work it out alone; early conversations tend to make later ones easier.