I. ˈnītˌma(a)](ə)r, -me], ]ə\ noun
Etymology: Middle English, from night (I) + mare (spirit)
1. : an evil spirit formerly thought to oppress people during sleep: as
a. : incubus
b. : succubus
c. : a hag sometimes believed to be accompanied by nine attendant spirits
the nightmare , with her whole ninefold, seems to make it the scene of her gambols — Washington Irving
2. : a frightening dream accompanied by a sense of oppression or suffocation that usually awakens the sleeper
3.
a. : something producing a feeling of burden, agitation, anxiety, or terror : a source of trouble or worry
the worst nightmares were the bridges high above rushing torrents — Dillon Ripley
the nightmare of the surgeon dealing with battle wounds is infection — C.L.Boltz
b. : apprehension , worry
the life of a hotel man here is precarious and full of nightmares — Sam Schneider
4. : an experience, situation, or work of imagination having the monstrous character of a nightmare
their existence would be one living nightmare of hideous watchfulness and dread — Blue Book
signs that we dwellers in the modern nightmare love one another — F.A.Swinnerton
an enormous imitation palace … a nightmare of pretentiousness — John Hersey
Synonyms: see fancy
II. adjective
1. : of or relating to a nightmare
a nightmare obsession in some current poetry
2. : nightmarish
began a nightmare existence in an iron lung — New York Times Book. Review
years that seemed to pass with nightmare speed — William DuBois