Hallucinations are where someone sees, hears, smells, tastes or feels things that don't exist outside their mind.
They're common in people with schizophrenia, and are usually experienced as hearing voices.
Hallucinations can be frightening, but there's usually an identifiable cause. For example, they can occur as a result of:
taking illegal drugs or alcohol
a mental illness, such as schizophrenia or dementia
a progressive neurological condition, such as Alzheimer's disease or Parkinson's disease