Night or nighttime is the period of time when the sun is below the horizon. The opposite of night is day (or "daytime" to distinguish it from "day" as used for a 24-hour period). The start and end times of night vary based on factors such as season, latitude, longitude and timezone.
At any given time, one side of the planet Earth is bathed in light from the Sun (the daytime) and the other side of the Earth is in the shadow caused by the Earth blocking the light of the sun. This shadow is what we call the darkness of night.