The time between the end of evening civil twilight and the start of morning civil sunrise is known as night. 14 CFR 61.109 refers to this term once more. As a result, a day is described as the time between the start of civil sunrise in the morning and the end of civil twilight in the evening.
According to my understanding, the FAA’s official accepted demarcation between daylight and night time is 14 CFR 1.1; however, it is not used as a standard when defining when night currency is applicable or when to use exterior aircraft lighting, primarily because ambient lighting at dusk and daybreak affects airman prowess during this time.