Learner's definition of NIGHTMARE
1 : a dream that frightens a sleeping person: a very bad dream
Mommy, I had a really scary nightmare.
She had a recurring nightmare [=she had the same bad dream many times] about losing her job.
2 : a very bad or frightening experience or situation
The party was a complete nightmare.
a nightmare situation/scenario
Losing a child is every parent's worst nightmare. [=the thing every parent fears most]
— nightmarish/ˈnaɪtˌmerɪʃ/adjective,[more nightmarish; most nightmarish]