Definition of nightmare noun from the Oxford Advanced American Dictionary

     

    nightmare

     noun
    noun
    NAmE//ˈnaɪtmɛr//
     
     
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  1. 1 a dream that is very frightening or unpleasant He still has nightmares about the accident. She has a recurring nightmare about being stuck in an elevator.
  2. 2nightmare (for somebody) an experience that is very frightening and unpleasant, or very difficult to deal with The trip turned into a nightmare when they both got sick. (informal) Nobody knows what's going on—it's a nightmare! (informal) Filling in all those forms was a nightmare. Losing a child is most people's worst nightmare. If it goes ahead, it will be the nightmare scenario (= the worst thing that could happen). a nightmare situation
nightmarish
 
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NAmE//ˈnaɪtˌmɛrɪʃ//
 
adjective nightmarish living conditions
See the Oxford Advanced Learner's Dictionary entry: nightmare