Webster's English Dictionary

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nightmare

night.mare \'ni-t-.ma(*)r, -.me(*)r\ \-ish\ n 1: an evil spirit formerly 
   thought to oppress people during sleep  2: a frightening dream accompanied 
   by a sense of oppression or suffocation  that usu. awakens the sleeper 3: 
   an experience, situation, or object having the monstrous character of a  
   nightmare or producing a feeling of anxiety or terror - night.mar.ish aj