The definition of a nightmare is a bad dream, or an experience that is like a bad dream.(noun)
See nightmare in Webster's New World College Dictionary
Origin: ME nihtmare: see night & mare
See nightmare in American Heritage Dictionary 4
Origin: Middle English, a female demon that afflicts sleeping people
Origin: : night, night; see night
Origin: + mare, goblin (from Old English; see mer- in Indo-European roots).
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