A hallucination is a sensory experience, while you are awake and conscious, without any corresponding cause in the external world which could be shared by others. So it would include hearing a voice when nobody is speaking and no-one else could hear it; or seeing a person or object which nobody else near you can see.
There can also be hallucinations of touch (such as feeling things crawling over your skin, when there are none); hallucinations of smell or taste occur, but less commonly, and may be more typical of the aura that warns that an epileptic fit is imminent.
Seeing or hearing things.
Psychotic illnesses such as schizophrenia, or very severe depression or bipolar disorder, or post-traumatic stress disorder. Sleep deprivation; fever; temporal lobe epilepsy, or the aura preceding an epileptic fit. Some brain tumours. Severe physical illness in which one is delirious, such as liver or kidney failure. Sensory deprivation in which one is, temporarily or permanently, blind or deaf. Intoxication by or withdrawal from various drugs and substances of abuse - such as cocaine, or amphetamine, alcohol (especially in delirium tremens).
Hallucinations may be entirely normal in bereavement, in the form of briefly seeing or hearing the loved person who has recently died - but these are brief, and the person experiencing them recognises that they are not real. Other normal hallucinations include hypnagogic hallucinations which can occur as one is falling asleep, and hypnopompic hallucinations which occur when one is waking up. These are brief, and one rapidly recognises that they don't represent reality.
Not safe until a cause has been established by a doctor, and is being properly treated. While this is being arranged, do not leave the person alone, speak calmly and soothingly, letting them know what is being done. Don't get over-anxious or talk behind their back.
When to see a doctor
When you recognise that this is what you are experiencing, or when family or friends tell you that this is what is happening to you. When someone in the family starts to hallucinate, arrange for them to see a doctor as soon as possible, and do not leave them alone.
What to expect at the doctor
A careful history and physical examination, perhaps blood and urine tests to check for a number of possible physical illnesses.
This, of course, depends on the cause of the hallucination. Treatment obviously needs, where possible, to focus on clearing up the cause of these experiences. Anti-psychotic medications may help to remove or at least reduce the disturbance caused by the presence of hallucinations.