a day is approximately the period of time during which the earth completes one rotation around its axis. a solar day is the length of time which elapses between the sun reaching its highest point in the sky two consecutive times. days on other planets are defined similarly and vary in length due to differing rotation periods, that of mars being slightly longer and sometimes called a sol.
in 1960, the second was redefined in terms of the orbital motion of the earth in the year 1900, and was designated the si base unit of time. the unit of measurement "day", was redefined as 86,400 si seconds and symbolized d. in 1967, the second and so the day were redefined by atomic electron transition. a civil day is usually 86,400 seconds, plus or minus a possible leap second in coordinated universal time (utc), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.
day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. however, its use depends on its context; for example, when people say 'day and night', 'day' will have a different meaning: the interval of light between two successive nights, the time between sunrise and sunset; the time of light between one night and the next. for clarity when meaning 'day' in that sense, the word "daytime" may be used instead, though context and phrasing often makes the meaning clear. the word day may also refer to a day of the week or to a calendar date, as in answer to the question, "on which day?" the life patterns (circadian rhythms) of humans and many other species are related to earth's solar day and the day-night cycle.
daytime image of the bay of naples, italy