Nightmare (noun)

1. A disturbing dream.
2. A situation or experience that is deeply distressing or traumatic.
3. An evil spirit supposed to oppress persons during sleep.

Origin:

Mid 16th century: from night + mare, originally in the sense "incubus, a terrifying dream or vision".

Examples:

1. She had a nightmare about falling from a tall building.
2. The war was a nightmare for the people who lived through it.
3. The traffic jam was a nightmare for the commuters.
4. The nightmare haunted him for weeks after it happened.
5. The accident was a nightmare that he could never forget.