Pronunciation (UK) IPA (key): /ˈnaɪt.mɛə/ (US) IPA (key): /ˈnaɪt.mɛɚ/ or [nʌɪʔ.mɛər] Audio (US)(file) Noun Singular nightmare Plural nightmares (countable) A nightmare is a bad or frightening dream. Related words nightmarish Wikiwand - on Seamless Wikipedia browsing. On steroids.Remove ads