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That is, is there a word for the 24-hour pe­riod start­ing at mid­night?

I know that tech­ni­cally a day means ei­ther 24 hours or the pe­riod of time be­tween sun­rise and sun­set, and that day and night would be specif­i­cally 24 hours. A side­real day is ap­prox­i­mately a lit­tle less than 24 hours, and a nych­the­meron is 24 hours – though it can start at any time.

Is there an equiv­a­lent of nych­the­meron that specif­i­cally starts at mid­night?

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    Why do you think it's not just a day?
    – KillingTime
    Commented Sep 11, 2021 at 2:14
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    The most technically correct verbiage that comes to my mind at present is "span of one calendar date" (which I realize is longer than a single word). Close in meaning is "calendar day", which could be thought-of as a singular word by hyphenating it (as "calendar-day"). @KillingTime ~~Absent lots of context, 'day' oft is too ambiguous; there exist three or more distinct meanings that it can convey. If aforementioned context makes it clear that the sub-kind of "day" must be of the 'gregorian calendar' variety, then omitting the modifier 'calendar' should be copacetic; otherwise, keep the lexeme.
    – 11qq00
    Commented Sep 11, 2021 at 3:11
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    If you are using the Gregorian and Greenwich Mean Time or one of its derivatives such as Eastern Standard Time in the US the days start at midnight local time and continue until a vanishingly small fraction of a second before the next midnight. That's why New Year celebrations include a countdown to midnight. In other time systems days start at different times, for instance in the Muslim Hijri calendar days start at sunset on what, to Europeans, is the day before. The same applies to the days of the Hebrew calendar which is why the Sabbath starts on Friday night.
    – BoldBen
    Commented Sep 11, 2021 at 8:22
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    First, are you asking about the Rata Die? Second, do you mean midnight this morning or midnight tonight? The 24 hours following midnight this morning we call today, while the 24 hours following midnight tonight we call tomorrow. We also have two separate words for the 24 hours preceding both midnights. :)
    – tchrist ♦
    Commented Sep 11, 2021 at 20:06
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    From all the answers and comments, it seems unclear as to what your restrictions are. 'Day' or 'today' seem to match your stated criteria perfectly fine. If you absolutely need to have the concept '24 hour period from midnight to midnight' then 'calendar day' is sufficiently specific. Note that it is possible to use more than one word in science writing to get a specific meaning.
    – Mitch
    Commented Sep 11, 2021 at 20:40

2 Answers 2

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According to the online version of Black’s Law Dictionary, a natural day is “properly the period of twenty-four hours from midnight to midnight. Co. Litt. 135; Fox v. Abel. 2 Conn. 541; People v. Hatch, 33 111. 137.”

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    '... Though sometimes taken to mean the “day-time” or time between sunrise and sunset.' And M-W gives conflicting definitions, so again a term variously defined and hence ambiguous.
    – Edwin Ashworth
    Commented Sep 11, 2021 at 16:33
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It can also be called a "day of the week" — which means one of the seven named days. (But not "weekday", which often is understood to exclude Saturday and Sunday.)

However, unless you specify exactly what you mean, any reference to a "day" is subject to ambiguity, and many people will not automatically assume that you mean it to begin at midnight.

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