Nychthemeron

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Nychthemeron /nɪkˈθɛmərɒn/, occasionally nycthemeron or nuchthemeron, is a period of 24 consecutive hours. It is sometimes used, especially in technical literature, to avoid the ambiguity inherent in the term day.

It is the period of time that a calendar normally labels with a date, although a nychthemeron simply designates a time-span that can start at any time, not just midnight.