horror vs nightmare

horror

noun
  • An intense anxiety or a nervous depression; often the horrors. 

  • An intense distressing emotion of fear or repugnance. 

  • A genre of fiction designed to evoke a feeling of fear and suspense. 

  • An individual work in this genre. 

  • Delirium tremens. 

  • A nasty or ill-behaved person; a rascal or terror. 

  • Intense dislike or aversion; an abhorrence. 

  • Something horrible; that which excites horror. 

nightmare

noun
  • Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure. 

  • A very bad or frightening dream. 

  • A feeling of extreme anxiety or suffocation experienced during sleep; Sleep paralysis. 

verb
  • To have a nightmare. 

How often have the words horror and nightmare occurred in a corpus of books? (source: Google Ngram Viewer )