Nychthemeron or nycthemeron or nuchthemeron (Greek νυχθήμερον from the words nykt– “night”, and (h)emera “day, daytime”) 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.
In other languages
The word etmaal in Dutch and the word døgn in Danish and Norwegian, or dygn in Swedish, can mean 24 hours, or more loosely a day plus a night in no particular order. Unlike a calendar date, only the length is defined, with no particular start or end. These words are basic and essential in these languages, and unlike “nychthemeron”, are not associated with jargon. Esperanto can use the synonyms diurno and tagnokto (“day-night”).