Phobia is an excessive, irrational and morbid fear of any object or situation which is apparently benign at that moment. But the person constantly avoids the feared object or situation.
Certain common phobias are claustrophobia (fear of closed spaces), acrophobia (fear of height), criscaidocophobia (fear of no.13).
Treatment is required when fear interferes with normal functioning of the individual. The person is gradually exposed to the feared object or situation until the threatening situation or object can be faced without fear (known as systemic desensitization).