(now rare) A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.

To have a nightmare.

plural of nightmare

(informal) A military situation for which it is difficult to plan or organize the logistics.

(temporal location) During a nightmare.

The middle part of a nightmare, by implication the scariest part, with no end in prospect; the state of being in a nightmare.

(idiomatic) terrifying, shocking

plural of logistical nightmare

(informal) Something capable of inducing nightmares, often by being highly disturbing.

Resembling or characteristic of a nightmare; nightmarish.