Nightmare
From Wikipedia, a free encyclopedia written in simple English for easy reading.
When people sleep, they sometimes have dreams. Very few people can remember what their dreams were last night. Some people have good dreams, and other people have bad dreams. A bad dream is called a nightmare.
If you look at where the word nightmare comes from, it means something like 'story of the night'. However, in old times (and even now) some people are afraid of the night. That is how a story of the night became a 'nightmare'.