means The time when daylight gives way to darkness as the sun sets and evening becomes night.

from A plain compound of two old English words doing exactly what they say: 'night' (from Old English 'niht', a Germanic word with cousins all across the family — German 'Nacht', Dutch 'nacht', and a deeper relation to Latin 'nox') plus 'fall', the same word for things descending. So nightfall is literally night 'falling' on the world — picturing darkness as something that drops over the land like a curtain. The figurative use of 'fall' for dusk is old, and the tidy compound itself is a later, mostly English coinage.