What is the meaning of Nightmare?

A very bad or frightening dream.

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

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

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

To experience a nightmare.

To imagine (someone or something) as in a nightmare.

To trouble (someone or something), as by a nightmare.

Source: wiktionary.org