In compounds probably means “darkness”, ultimately from Proto-Indo-European *nébʰos (“cloud”) whence the Proto-Germanic *nebulaz, cognates with Old High German nebul (“fog”) (New High german nebel, whence Nebel (“fog, mist, haze; nebula”)) and Latin nebula (“fog; cloud; vapor”).
Confer njól (“poetic: night”).
Derived terms 
- niflfarinn (poetic: gone to hell, dead)
- niflgóður (poetic: evil)
- Niflheimur (Niflheim; the Mist Home, the "Abode of Mist", the Mist World; the domain of the dead, the Underworld)
- Niflhel (poetic: the dark abode of the being Hel)
- niflungur (poetic: a king, a head of state)
- Niflungar (Nibelung)
- niflvegur (poetic: a dark road)