Nightfall is the part of a day when the sun goes down and evening begins. In suburban neighborhoods, parents often call their kids to come home for dinner at nightfall.
It gets dark at nightfall, marking the end of the day. You can also all this time dusk or twilight. If your dad warns you to be home from your bike ride by nightfall, he wants you home before sunset. Nightfall has a bit of an old fashioned, poetic sound, and it dates from the early eighteenth century, a combination of night, from the Old English niht, or "darkness," and fall, or "come suddenly."