A "day" is a unit of time that represents the period it takes for the Earth to complete one full rotation on its axis. This rotation causes the cycle of daylight and darkness, with one side of the Earth facing the Sun experiencing daylight, while the opposite side is in darkness. A standard day is divided into 24 hours, which are further divided into minutes and seconds. Days are commonly used to measure time in various contexts, such as in calendars and schedules. The concept of a day can vary slightly depending on the Earth's rotation and its orbit around the Sun, leading to variations like leap years and daylight saving time adjustments.