두 단어의 구분은 개념상 포함하고 안 하고의 차이인 것 같습니다.

   twilight 은 새벽녁과 저녁때 어둑어둑할 때(노을 질 때), 완전 밝지도 완전 어둡지도 않은 때를

말하고, nightfall 은 낮이 끝나고 밤이 시작되는 때를 말합니다.

twilight (plural twilights)

1. The soft light in the sky seen before the rising and (especially) after the setting of the sun, occasioned by the illumination of the earth’s atmosphere by the direct rays of the sun and their reflection on the earth. I could just make out her face in the twilight.
2. The time when this light is visible; the period between daylight and darkness. It was twilight by the time I got back home.
3. Any faint light through which something is seen; an in-between or fading condition. The twilight of probability. —Locke.

nightfall (uncountable)

1. The close of the day; the coming of night.