Day

ddiurnaldays24 hoursdailysolar daytotal dayscivild −1 Day School
A day is approximately the period of time during which the Earth completes one rotation around its axis.wikipedia
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Second

ssecmegasecond
In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time.
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

Solar time

mean solar timesolar dayapparent solar time
A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.
The fundamental unit of solar time is the day.

Midnight

MidniteZero Hour12:00
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. A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination).
Midnight is the transition time from one day to the next – the moment when the date changes.

Week

weeklyseven-day weekweek number
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?"
A week is a time unit equal to seven days.

Daytime

dayday lengthdaytime hours
However, in order to be clear when using 'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period; this is since daytime usually means 'the time of the day between sunrise and sunset.
On Earth, daytime is roughly the period of the day during which any given point in the world experiences natural illumination from especially direct sunlight.

Night

nighttimenight timeovernight
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.
The opposite of night is day (or "daytime", to distinguish it from "day" referring to a 24-hour period).

Year

myaMay
The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude as well as latitude), and the time of year (as indicated by ancient hemispherical sundials).
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.

SI base unit

base unitSI base unitsbase units
In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time.

Hour

htime of dayhr
An hour (symbol: h; also abbreviated hr.) is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

Nychthemeron

24 hours
It is sometimes used, especially in technical literature, to avoid the ambiguity inherent in the term day.

Tidal acceleration

tidal frictiontidal decelerationtidal heating
Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in minor variations for both solar days and stellar "days".
The average tidal bulge is synchronized with the Moon's orbit, and Earth rotates under this tidal bulge in just over a day.

Metric time

centidaymetrication of the dayMilliday
The most successful alternative is the centiday, equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1 000 seconds) and equal to the traditional Chinese unit, kè.
Other units of time: minute, hour, and day, are accepted for use with SI, but are not part of it.

Sunrise

rising sunrisingrises
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.

Traditional Chinese timekeeping

double-hour of the dayKe
The most successful alternative is the centiday, equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1 000 seconds) and equal to the traditional Chinese unit, kè.
This system used two standards to measure the time in a solar day.

International System of Units

SISI unitsSI unit
A day, symbol d, defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI.

Noon

solar noonMiddayMid-day
A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination).
These four 24-hour days occur in both hemispheres simultaneously.

Orders of magnitude (time)

kilosecondyoctosecondpetasecond
The most successful alternative is the centiday, equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1 000 seconds) and equal to the traditional Chinese unit, kè.

Morning

Morningssmall hourswee hours
Morning precedes afternoon, evening, and night in the sequence of a day.

24-hour clock

24-hour24-hour notation24-hour format
The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23.

Holiday

holidaysreligious holidayHoly Days
Days such as Christmas Eve, Halloween, and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening.
The word holiday comes from the Old English word hāligdæg (hālig "holy" + dæg "day").

Daylight

111,000blue skyday
For an explanation of why it is not exactly half, see here).

Mercury (planet)

MercuryMercurioplanet Mercury

Earth's rotation

rotation of the EarthrotationEarth rotates
A day is approximately the period of time during which the Earth completes one rotation around its axis.

Time

temporaldurationsequence of events
In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time.

Unit of measurement

unitunits of measurementweights and measures
The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d.