1 a frightening or unpleasant dream.
2 colloq. a terrifying or very unpleasant experience or situation.
3 a haunting or obsessive fear.
Derivatives:
nightmarish adj. nightmarishly adv.
Etymology: an evil spirit (incubus) once thought to lie on and suffocate sleepers: OE maeligre incubus
Useful english dictionary. 2012.